Accessor: add methods to write or clear CRS info

[benbovy]

Add these two methods (and their DataArray equivalents):

• Dataset.proj.write_crs_info()
• Dataset.proj.clear_crs_info()

It is possible to customize how a CRS is translated into coordinate attributes by passing a callable to .proj.write_crs_info(). Two builtin callables are provided:

• format_compact_cf: just write a crs_wkt attribute (default)
• format_full_cf_gdal: write crs_wkt, spatial_ref and all CF grid mapping variable attributes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.37%. Comparing base (66d3c7d) to head (87b6516).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   98.21%   98.37%   +0.16%     
==========================================
  Files           9       10       +1     
  Lines         559      614      +55     
==========================================
+ Hits          549      604      +55     
  Misses         10       10              

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