tox.ini: find Notebooks to run with nbval-lax using unix find cmd (WIP)

[willfurnass]

Closes #325

[codecov-io]

Codecov Report

Merging #363 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
- Coverage   43.02%   42.77%   -0.25%     
==========================================
  Files          10       10              
  Lines        9295     9307      +12     
  Branches     2520     2520              
==========================================
- Hits         3999     3981      -18     
+ Misses       4886     4852      -34     
- Partials      410      474      +64

Impacted Files	Coverage Δ
mumot/utils.py	65.5% <0%> (-4.53%)	⬇️
mumot/controllers.py	74.61% <0%> (-3.25%)	⬇️
mumot/models.py	69.33% <0%> (-2.65%)	⬇️
mumot/__init__.py	92.3% <0%> (-0.29%)	⬇️
mumot/views.py	67.31% <0%> (+1.05%)	⬆️

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