Unify logic for computing vmin and vmax in ribs.visualize

[btjanaka]

Description

The logic for computing vmin and vmax for an archive is not trivial. The logic for computing each of vmin and vmax depends on factors including:

1. Whether vmin is None or passed in
2. Whether vmax is None or passed in
3. Whether the objectives array extracted from the archive has len(0) (i.e., is empty) or has values

Currently, the logic for handling vmin and vmax is repeated across all the functions in ribs/visualize, and there are gaps for the cases where vmin is None and vmax is not None, and vice versa. This PR fills in that logic and moves it into a common utility in ribs/visualize/_utils.py.

TODO

• Add compute_vmin_vmax util in ribs/visualize/_utils.py
• Add tests in tests/utils/utils_test.py
• Update visualize functions under ribs/visualize/ to use the new util
  • Update cvt_archive_heatmap and cvt_archive_3d_plot -- they seem to have more involved logic -> It turns out they had a default separation of 0.01 when the objectives were equal, while we use 0.1 -- hence, the baseline images for test_vmin_equals_vmax changed for both these functions
• Ensure that existing tests pass -> Some baseline images changed slightly
• Add tests for 1-2 visualize functions that test various settings of vmin and vmax when the archive is empty -> cvt_archive_heatmap

Status

• I have read the guidelines in
  CONTRIBUTING.md
• I have linted and formatted my code with ruff and pylint
• I have tested my code by running pytest
• I have added a description of my change to the changelog in HISTORY.md
• This PR is ready to go