Add a 'solventmask' keyword to 'closest'

[drroe]

Because why not. Can be a pain to redefine what solvent is for the entire system just for a single command.

-Dan

[hainm]

+1, it's good for mixed solvent.

[DDGmichigan]

Really eager to get the solventmask utility in the closest action command!

[drroe]

Addressed by #725.