Ensure can create models from a string instead of a cell reference

[willfurnass]

Directly addresses #156. Could also help with #157 (measuring code coverage during Notebook runs).

[codecov-io]

Codecov Report

Merging #200 into master will increase coverage by 37.61%.
The diff coverage is 87.5%.

@@             Coverage Diff             @@
##           master     #200       +/-   ##
===========================================
+ Coverage   21.21%   58.82%   +37.61%     
===========================================
  Files           6        6               
  Lines        8376     8377        +1     
  Branches     1768     1769        +1     
===========================================
+ Hits         1777     4928     +3151     
+ Misses       6556     2977     -3579     
- Partials       43      472      +429

Impacted Files	Coverage Δ
mumot/__init__.py	66.9% <87.5%> (+58.22%)	⬆️
mumot/gen/PSParser.py	42% <0%> (+3.47%)	⬆️
mumot/process_latex/process_latex.py	23.84% <0%> (+7.17%)	⬆️

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