feat(contours): use standard contours for StructuredGrid map view plots

[wpbonelli]

• switch from matplotlib tricontour to contour for StructuredGrid map view plots
  • requested in feature: use matplotlib contour for StructuredGrid #1605
  • moved from fix(multiple): miscellanous fixes/enhancements #1614
  • code rearranged a bit to separate the tri / contours cases

[codecov]

Codecov Report

Merging #1615 (1a574a1) into develop (5a9bec2) will decrease coverage by 0.0%.
The diff coverage is 64.5%.

@@            Coverage Diff            @@
##           develop   #1615     +/-   ##
=========================================
- Coverage     72.7%   72.7%   -0.1%     
=========================================
  Files          251     251             
  Lines        54501   54501             
=========================================
- Hits         39671   39651     -20     
- Misses       14830   14850     +20     

Impacted Files	Coverage Δ
flopy/plot/map.py	82.3% <64.5%> (-1.9%)	⬇️
flopy/utils/get_modflow.py	64.0% <0.0%> (-1.5%)	⬇️
flopy/utils/geometry.py	74.5% <0.0%> (-1.3%)	⬇️
flopy/export/utils.py	65.4% <0.0%> (-0.3%)	⬇️

[mbakker7]

I just tested this PR on my models that caused problems before and it works great.
It gets my approval if that is worth anything.
Can this PR be approved and merged (teaching a class in 2 weeks and would be great when there is a new flopy).