Add force-save to Otter Assign for R notebooks

[chrispyles]

Closes #474

[codecov]

Codecov Report

Patch coverage: 81.25% and project coverage change: -0.02% ⚠️

Comparison is base (3fdf05f) 80.24% compared to head (510b739) 80.23%.
Report is 1 commits behind head on beta.

Additional details and impacted files

@@            Coverage Diff             @@
##             beta     #675      +/-   ##
==========================================
- Coverage   80.24%   80.23%   -0.02%     
==========================================
  Files          62       62              
  Lines        4029     4037       +8     
==========================================
+ Hits         3233     3239       +6     
- Misses        796      798       +2     

Files Changed	Coverage Δ
otter/assign/notebook_transformer.py	92.02% <ø> (+0.39%)	⬆️
otter/assign/assignment.py	95.20% <50.00%> (-1.50%)	⬇️
otter/assign/r_adapter/cell_factory.py	90.24% <90.90%> (-0.94%)	⬇️
otter/generate/__init__.py	86.25% <100.00%> (ø)

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