fix(gridgen): only add suffix for generated shpfiles

[wpbonelli]

• close bug: utils\gridgen grid refinement with shapefile #2021
• don't add ".shp" extension if caller provides shapefile name
• improve features parameter handling and docstring

[codecov]

Codecov Report

Merging #2022 (ee025c0) into develop (c63bfca) will increase coverage by 0.0%.
The diff coverage is 83.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2022   +/-   ##
=======================================
  Coverage     72.8%   72.8%           
=======================================
  Files          259     259           
  Lines        59040   59042    +2     
=======================================
+ Hits         43004   43006    +2     
  Misses       16036   16036           

Files	Coverage Δ
flopy/utils/gridgen.py	86.8% <83.3%> (+2.3%)	⬆️

... and 5 files with indirect coverage changes