Add pyvista-to-openep converter

[p-j-smith]

Changes made:

• added new module openep.converters
• added function openep.converters.pyvista_converters.from_pyvista - to convert a pyvista.PolyData mesh to an openep.Case dataset. Note - currently, no point, cell, or field data is extracted from the mesh
• added function openep.converters.pyvista_converters.to_pyvista. Equivalent to case.create_mesh but with the added option of adding scalar fields to the created mesh as point data (stored in mesh.point_data)

[codecov-commenter]

Codecov Report

Merging #169 (c81f6ce) into dev (4deb0d2) will decrease coverage by 0.53%. The diff coverage is 38.46%.

@@            Coverage Diff             @@
##              dev     #169      +/-   ##
==========================================
- Coverage   58.63%   58.09%   -0.54%     
==========================================
  Files          12       13       +1     
  Lines         950      976      +26     
  Branches      144      145       +1     
==========================================
+ Hits          557      567      +10     
- Misses        372      388      +16     
  Partials       21       21              

Impacted Files	Coverage Δ
openep/data_structures/case.py	52.38% <ø> (ø)
openep/converters/pyvista_converters.py	38.46% <38.46%> (ø)

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