Add custom attribute support to mmCIF files

[javisanchezdelrio@…]

Inspired by post on chimerax-users mailing list.

Adding support for custom attributes in mmCIF files needs:

• a way to choose which custom attributes to write
  • could be automatic and limited to attributes created with setattr
  • might want to allow for built-in custom attributes
• an optional floating point precision
  • could be used in Render/Select by Attribute dialog too
• a table that lists the custom attributes to be read
  • ie., setattr information