mfgrdfile: export follow-on

[mjreno]

Follow-up to #2711:

• all export() params except filename are keyword-only for forward looking compatibility
• validate crs input strings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (eff4582).
⚠️ Report is 180 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2779      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132729    +8594     
===========================================
+ Hits         68947    96361   +27414     
+ Misses       55188    36368   -18820     

Files with missing lines	Coverage Δ
flopy/mf6/utils/binarygrid_util.py	89.6% <100.0%> (-1.2%)	⬇️

... and 579 files with indirect coverage changes

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