Add suggestions for xarray .open_mfdataset parameters

[ppinchuk]

By default xarray will attempt to validate coordinates during concatenation. Since rex-style data coordinates are known to match, it;s better to skip this validation step in order to get a huge performance gain when using xarray .open_mfdataset. This PR adds some notes about this and a warning to the open_mfdataset_hsfs function

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.70%. Comparing base (74e8acf) to head (1fc05e8).

Files with missing lines	Patch %	Lines
rex/external/rexarray.py	0.00%	3 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
- Coverage   67.73%   67.70%   -0.03%     
==========================================
  Files          40       40              
  Lines        7095     7098       +3     
  Branches     1039     1040       +1     
==========================================
  Hits         4806     4806              
- Misses       1961     1964       +3     
  Partials      328      328              

Flag	Coverage Δ
unittests	67.70% <0.00%> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
rex/external/rexarray.py	73.52% <0.00%> (-0.58%)	⬇️

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