Fix resample_df for Pandas Series

[jgieseler]

• Fix resample_df( [..] cols_unc='auto') so that it works with Pandas Series
• Fix NaN not correctly filter in STEREO/MAGPLASMA dataset
• Fix some tests

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.51%. Comparing base (18cb27a) to head (93c5af5).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
seppy/util/__init__.py	76.92%	3 Missing ⚠️
seppy/loader/stereo.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   73.29%   73.51%   +0.21%     
==========================================
  Files          15       15              
  Lines        2887     2892       +5     
==========================================
+ Hits         2116     2126      +10     
+ Misses        771      766       -5     

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