Add access for vtkToTensorMesh function

[banesullivan]

The code was already in discretize, these changes just make the function accessible to users to convert vtkRectilinearGrid objects to TensorMesh objects.

Needed this recently for a project - it was easiest to create my mesh in vtki and repeatedly visualize it while tweaking its size. Then I could convert it to discretize to use in an inversion.

Example

For example (needs to be done in IPython)

import vtki
from vtki import examples
import discretize
import numpy as np

# Create a plotting window
p = vtki.BackgroundPlotter()
p.show_grid()

# Get a sample topo surface from vtki
# Note: this requires vtki>=0.17.1
topo = examples.download_st_helens().warp_by_scalar()

p.add_mesh(topo, name='topo')

# Create the mesh interactively
# tweak these parameters and rerun this cell until satisfied

b = topo.bounds
xcoords = np.linspace(b[0], b[1], 50)
ycoords = np.linspace(b[2], b[3], 50)
zcoords = np.linspace(b[4]-5000, b[5], 50)

mesh = vtki.RectilinearGrid(xcoords, ycoords, zcoords)

p.add_mesh(mesh, name='mesh', opacity=0.5, show_edges=True)
# output the mesh
mesh

RectilinearGrid	Information
N Cells	117649
N Points	125000
X Bounds	5.579e+05, 5.677e+05
Y Bounds	5.108e+06, 5.122e+06
Z Bounds	-3.636e+03, 3.225e+03
Volume	9.381e+11
N Scalars	0

# Once satisfied, convert to discretize:
dmesh, _ = discretize.TensorMesh.vtkToTensorMesh(mesh)
dmesh

<discretize.TensorMesh.TensorMesh at 0xb2a555588>

And a GIF to demo

[codecov]

Codecov Report

Merging #140 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   72.75%   72.76%   +0.01%     
==========================================
  Files          20       20              
  Lines        4525     4527       +2     
==========================================
+ Hits         3292     3294       +2     
  Misses       1233     1233

Impacted Files	Coverage Δ
discretize/mixins/vtkModule.py	81.9% <100%> (+0.17%)	⬆️

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[lheagy]

🎉 Nice work @banesullivan - many thanks for the GIF :)