feat(vtk): add to_pyvista() method

[wpbonelli]

Add to_pyvista() method to the Vtk class as suggested here. Returns up to 3 PyVista meshes:

• underlying modelgrid
• polygons (e.g. for HFB)
• particle tracking pathlines

Only meshes which have been populated with add_model(), add_pathline_points(), etc are returned. If only one mesh has been configured, it is returned alone. If zero or >1 meshes have been configured, the return value is a list. This allows similar usage for 1+ meshes via tuple dereferencing

from flopy.export.vtk import Vtk

vtk = Vtk(model=gwf, binary=False, smooth=False)

# given model 'gwf'
vtk.add_model(gwf)
grid = vtk.to_pyvista()

# given recarray or list of recarrays 'pl'
vtk.add_pathline_points(pl)
grid, pathlines = vtk.to_pyvista()

Not sure this is the right approach — other options could be

• independent methods for each mesh, e.g. grid_to_pyvista(), polygons_to_pyvista()
• a parameter to select the mesh with default value "grid", e.g. to_pyvista(), to_pyvista(mesh="pathlines")

A similar to_pyvista() method may also be convenient on Grid types

[codecov]

Codecov Report

Merging #1771 (608aae3) into develop (7db9263) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           develop   #1771   +/-   ##
=======================================
  Coverage     72.1%   72.1%           
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  Files          254     254           
  Lines        55914   55919    +5     
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+ Hits         40364   40369    +5     
  Misses       15550   15550           

Impacted Files	Coverage Δ
flopy/export/vtk.py	70.2% <100.0%> (+0.1%)	⬆️

[mwtoews]

Looks great, thanks!