fix(shapefile_utils): check attr.array for None in model_attributes_to_shapefile

[wpbonelli]

• fix bug: shapefile_utils export fail with disu export #1775
• if array data (e.g. angldegx) is missing, a warning is shown but export proceeds
• add some test cases for above, factor out a function to create a sim with a GWF model on a DISU grid
• update test_export.py::test_freyberg_export to check shapefiles exported for each level (model/pkg/obj) exist
• update test_export.py::test_export_output to write netCDF files to tempdir instead of cwd

[codecov]

Codecov Report

Merging #1785 (bc93381) into develop (badc431) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           develop   #1785   +/-   ##
=======================================
  Coverage     72.1%   72.1%           
=======================================
  Files          254     254           
  Lines        55929   55930    +1     
=======================================
+ Hits         40377   40380    +3     
+ Misses       15552   15550    -2     

Impacted Files	Coverage Δ
flopy/export/shapefile_utils.py	87.8% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

[langevin-usgs]

Looks good, @w-bonelli. I say merge it in.