Add LabeledCircle to Mechanics.asy and base Mass and Charge off of it.

Add LabeledCircle to Mechanics.asy and base Mass and Charge off of it.

This makes label handling easy and consistent (I've also left more
label processing to the Label itself).  Unfortunately, I can't figure
out how to make subclasses in Asymptote, so you'll have to update your
code to access the parameters that moved into LabeledCircle.  There
are new .center() and .set_center() methods to future proof you
against further changes to the location of that parameter.