mf6: update dfns and mf6 module

[wpbonelli]

Add dfns and generate mf6 module for compatibility with new mf6. Leave out surface water models.

[codecov]

Codecov Report

Attention: Patch coverage is 97.26027% with 14 lines in your changes missing coverage. Please review.

Project coverage is 76.4%. Comparing base (bb9824e) to head (12d3670).
Report is 21 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/modflow/mfgwedisv.py	58.3%	5 Missing ⚠️
flopy/mf6/modflow/mfgwedisu.py	86.6%	2 Missing ⚠️
flopy/mf6/modflow/mfgwemve.py	80.0%	2 Missing ⚠️
flopy/mf6/modflow/mfgwfgnc.py	71.4%	2 Missing ⚠️
flopy/mf6/modflow/mfgwfmvr.py	83.3%	2 Missing ⚠️
flopy/mf6/modflow/mfgnc.py	85.7%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2399     +/-   ##
=========================================
+ Coverage     68.4%   76.4%   +7.9%     
=========================================
  Files          294     294             
  Lines        59390   59704    +314     
=========================================
+ Hits         40652   45619   +4967     
+ Misses       18738   14085   -4653     

Files with missing lines	Coverage Δ
flopy/mf6/modflow/__init__.py	100.0% <100.0%> (ø)
flopy/mf6/modflow/mfems.py	100.0% <100.0%> (+22.2%)	⬆️
flopy/mf6/modflow/mfgwe.py	100.0% <100.0%> (+58.8%)	⬆️
flopy/mf6/modflow/mfgweadv.py	100.0% <100.0%> (+30.0%)	⬆️
flopy/mf6/modflow/mfgwecnd.py	100.0% <100.0%> (+44.4%)	⬆️
flopy/mf6/modflow/mfgwectp.py	100.0% <100.0%> (+53.8%)	⬆️
flopy/mf6/modflow/mfgwedis.py	100.0% <100.0%> (+55.1%)	⬆️
flopy/mf6/modflow/mfgweesl.py	100.0% <100.0%> (+53.8%)	⬆️
flopy/mf6/modflow/mfgweest.py	100.0% <100.0%> (+43.4%)	⬆️
flopy/mf6/modflow/mfgwefmi.py	100.0% <100.0%> (+35.7%)	⬆️
... and 95 more

... and 147 files with indirect coverage changes