RFE updating molecular surface color

[Elaine Meng]

As mentioned in (closed) ticket #383:
"When a molecular surface changes shape it now becomes a single color which is the average of its previous vertex colors. If the surface was a single color then it keeps that single color. It would be better if the surface were colored on a per-atom basis (ie each atom patch has one color) that it keep that coloring."

I'm not suggesting this is urgent, just wanted to note it as a possible future enhancement.

[Tom Goddard]

This could be done but is a bit of work. Molecular surfaces would need to remember per-atom surface colors, sessions would need to save those, coloring by other methods (e.g. single color or by map value) would need to forget per-atom colors. Should be done though.

[Tom Goddard]

Fixed.

Made molecular surfaces remember per-atom coloring. I moved a lot of code around to do this, tested many coloring cases, but might have broken some surface coloring things -- look out.