WIP: SmilesWidget: Use MMFF94 to optimize generated structures

[danielhollas]

The wisdom seems to be that MMFF94 should be generally better than UFF. I've been using it for some time and did not notice major issues, I am proposing to use it as a default, with UFF as fallback.

TODO: This is a Draft. I still need to add error handling and tests.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.76%. Comparing base (2fa62c5) to head (33c48d3).

Files with missing lines	Patch %	Lines
aiidalab_widgets_base/structures.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   84.75%   84.76%           
=======================================
  Files          18       18           
  Lines        3365     3367    +2     
=======================================
+ Hits         2852     2854    +2     
  Misses        513      513           

Flag	Coverage Δ
python-3.12	84.73% <37.50%> (+<0.01%)	⬆️
python-3.9	84.75% <50.00%> (+<0.01%)	⬆️

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