[BUG] Fix various PendingDeprecationWarning and FutureWarning

[talgalili]

The list to fix (based on github flow)

=============================== warnings summary ===============================
tests/test_cli.py: 110 warnings
  /home/runner/work/balance/balance/balance/weighting_methods/ipw.py:278: RuntimeWarning:
  
  invalid value encountered in scalar divide

tests/test_cli.py::TestCli::test_cli_sep_input_works
  /home/runner/work/balance/balance/tests/test_cli.py:433: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_cli.py::TestCli::test_cli_sep_works
  /home/runner/work/balance/balance/tests/test_cli.py:363: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_cli.py::TestCli::test_cli_short_arg_names_works
  /home/runner/work/balance/balance/tests/test_cli.py:504: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:877: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:878: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:880: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:886: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:890: FutureWarning:
  
  Calling int on a single element Series is deprecated and will raise a TypeError in the future. Use int(ser.iloc[0]) instead

tests/test_sample.py::TestSample_metrics_methods::test_Sample_keep_only_some_rows_columns
  /home/runner/work/balance/balance/tests/test_sample.py:893: FutureWarning:
  
  Calling float on a single element Series is deprecated and will raise a TypeError in the future. Use float(ser.iloc[0]) instead

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test__prepare_weighted_stat_args
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:160: PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test__prepare_weighted_stat_args
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:189: PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test_weighted_mean
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:286: PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test_weighted_quantile
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:[443](https://github.com/facebookresearch/balance/actions/runs/12593610597/job/35099888317?pr=96#step:5:444): PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test_weighted_quantile
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:451: PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_stats_and_plots.py::TestBalance_weighted_stats::test_weighted_quantile
  /home/runner/work/balance/balance/tests/test_stats_and_plots.py:459: PendingDeprecationWarning:
  
  the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.

tests/test_weighted_comparisons_plots.py::Test_weighted_comparisons_plots::test_plot_dist
  /home/runner/work/balance/balance/tests/test_weighted_comparisons_plots.py:275: DeprecationWarning:
  
  This function is deprecated. Please call randint(11111, 11114 + 1) instead

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html