fix(modelgrid): retain crs data from classic nam files

[mwtoews]

This fixes a bug from #1737 where loading classic NAM files does not retain any CRS information that may be been stored in the header comment.

• With pyproj installed, the modelgrid properties .proj4 and .epsg are generated from the crs inputs.
• Without pyproj installed, .proj4 retains the original string input to crs, and .epsg retains a valid integer parsed from the crs input.

This PR adds tests for flopy.utils.mfreadnam.attribs_from_namfile_header and checks the same simulations with Modflow.load to inspect a few modelgrid properties. This PR was also tested locally without pyproj.

[codecov]

Codecov Report

Merging #1904 (b0e2a19) into develop (70b9a37) will increase coverage by 0.3%.
Report is 11 commits behind head on develop.
The diff coverage is 64.0%.

@@            Coverage Diff            @@
##           develop   #1904     +/-   ##
=========================================
+ Coverage     72.2%   72.6%   +0.3%     
=========================================
  Files          257     257             
  Lines        56488   56237    -251     
=========================================
+ Hits         40836   40840      +4     
+ Misses       15652   15397    -255     

Files Changed	Coverage Δ
flopy/mbase.py	69.2% <50.0%> (-0.1%)	⬇️
flopy/discretization/grid.py	75.3% <60.0%> (+0.2%)	⬆️
flopy/modflow/mf.py	69.0% <100.0%> (+<0.1%)	⬆️
flopy/utils/mfreadnam.py	34.6% <100.0%> (+0.5%)	⬆️

... and 9 files with indirect coverage changes

[aleaf]

Hi @mwtoews thanks for submitting this. It will be good to have this functionality preserved for loading and locating older models with minimal effort.