fix(mfdataplist): fix multiple issues

[wpbonelli]

Fix some cases where MFPandasList behaved inconsistently with MFList/MFTransientList

1. support None period data, fixed in set_record (added record["data"] is not None)
2. convert 0- to 1-based indexing for non-cell-ID fields marked with numeric_index
3. support external file paths via open/close, fixed with new branch in set_data()

These were flushed out by MODFLOW-ORG/modflow6#2868, which added # package-type stress-package to a number of DFNs. Since the pandas alternative list impl was added in #1955, list blocks in stress packages use MFPandasList by default while other packages use the old MFList, due evidently to some gaps in the pandas approach.

Also some related refactoring/fixes/cleanup

• support DIS2D/DISV1D/DISV2D cell IDs
• add _get_cellid_columns() helper to centralize cellid column logic
• delete _unique_column_name(), _untuple_manually()
• fix a typo in mffileaccess.py (cellid_nodes → cellid_node)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (8d13e27).
⚠️ Report is 176 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/data/mfdataplist.py	67.5%	25 Missing ⚠️
flopy/mf6/data/mffileaccess.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2775      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132637    +8502     
===========================================
+ Hits         68947    96277   +27330     
+ Misses       55188    36360   -18828     

Files with missing lines	Coverage Δ
flopy/mf6/data/mffileaccess.py	72.6% <0.0%> (-3.4%)	⬇️
flopy/mf6/data/mfdataplist.py	71.5% <67.5%> (-7.3%)	⬇️

... and 578 files with indirect coverage changes

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