Add style testing

[jcapriot]

In a similar manner to SimPEG, add flake8 testing

[codecov]

Codecov Report

Merging #300 (a2fd228) into main (eda1e5c) will increase coverage by 0.00%.
The diff coverage is 90.66%.

@@           Coverage Diff           @@
##             main     #300   +/-   ##
=======================================
  Coverage   85.22%   85.22%           
=======================================
  Files          38       38           
  Lines       11274    11281    +7     
=======================================
+ Hits         9608     9614    +6     
- Misses       1666     1667    +1     

Impacted Files	Coverage Δ
discretize/View.py	0.00% <0.00%> (ø)
discretize/_extensions/tree_ext.pyx	84.44% <ø> (ø)
discretize/base/base_mesh.py	93.01% <ø> (ø)
discretize/mixins/__init__.py	100.00% <ø> (ø)
discretize/mixins/vtk_mod.py	87.85% <0.00%> (ø)
discretize/tests.py	79.11% <ø> (ø)
discretize/utils/codeutils.py	0.00% <0.00%> (ø)
discretize/utils/coordinate_utils.py	88.88% <ø> (ø)
discretize/utils/coordutils.py	0.00% <0.00%> (ø)
discretize/utils/curvutils.py	0.00% <0.00%> (ø)
... and 22 more

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