fix(get_transmissivities): handle single r/c

[wpbonelli]

Apply np.atleast_1d() to r and c for consistency with x and y. Previously, passing single values for r and c resulted in IndexError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (086f8d3).
⚠️ Report is 117 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2697      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130394    +6259     
===========================================
+ Hits         68947    94378   +25431     
+ Misses       55188    36016   -19172     

Files with missing lines	Coverage Δ
flopy/utils/postprocessing.py	80.1% <100.0%> (+2.5%)	⬆️

... and 562 files with indirect coverage changes

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