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

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ccfcf80) 79.97% compared to head (9db58a5) 79.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   79.97%   79.98%   +0.01%     
==========================================
  Files          27       27              
  Lines        3830     3832       +2     
==========================================
+ Hits         3063     3065       +2     
  Misses        767      767              

Flag	Coverage Δ
python-3.10	79.98% <37.50%> (+0.01%)	⬆️
python-3.8	80.02% <37.50%> (+0.01%)	⬆️
python-3.9	80.02% <37.50%> (+0.01%)	⬆️

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Files	Coverage Δ
aiidalab_widgets_base/structures.py	78.06% <37.50%> (+0.06%)	⬆️

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