fix(mfdataarray): allow external arrays with unknown dimension, fix tas kper indexing

[wpbonelli]

• Handle time-indexed and period-indexed transient array data independently. TAS data uses float keys (0.0, 6.0, 12.0) representing actual simulation times. Standard transient data uses integer keys (0, 1, 2) representing stress period indices.
• Allow setting array data of unknown shape like TAS external.

Fix #2702.

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (4130606).
⚠️ Report is 124 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/data/mfdataarray.py	93.5%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2703      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      669      +25     
  Lines       124135   130536    +6401     
===========================================
+ Hits         68947    94584   +25637     
+ Misses       55188    35952   -19236     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdataarray.py	61.8% <93.5%> (-5.3%)	⬇️

... and 564 files with indirect coverage changes

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