fix(crosssection): make view x/y override symmetric, validate input

[wpbonelli]

little fixes for #2782

• only view="x" could force a direction, view="y" couldn't
• make invalid values an error instead of silently using "x"
• state that view has no effect for row/column-based cross sections in the docstring

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.7%. Comparing base (556c088) to head (2126aed).
⚠️ Report is 199 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2805      +/-   ##
===========================================
+ Coverage     55.5%    72.7%   +17.2%     
===========================================
  Files          644      661      +17     
  Lines       124135   133026    +8891     
===========================================
+ Hits         68947    96832   +27885     
+ Misses       55188    36194   -18994     

Files with missing lines	Coverage Δ
flopy/plot/crosssection.py	64.7% <100.0%> (-2.4%)	⬇️

... and 582 files with indirect coverage changes

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