Some tweaks to atom colors

[pfebrer]

This PR adds some changes to atom colors:

• There is one base class AtomColors, which you can subclass to provide a given color palette. What previously was AtomDefaultColors is now JMolColors. So now there is room for other color palettes.
• The hydrogen atom color was set to pure white, which was invisible on a white background. Now it is a pale shade of gray.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7f9b4dc) 85.78% compared to head (317ca3d) 85.77%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #684      +/-   ##
==========================================
- Coverage   85.78%   85.77%   -0.01%     
==========================================
  Files         367      367              
  Lines       43904    43917      +13     
==========================================
+ Hits        37662    37669       +7     
- Misses       6242     6248       +6     

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