SetFixAtom and SetIgnoreAtom


The Draw and AutoOpt tool now use OBForceField::(Un)Set***Atom() to
fix and ignore atoms. For the AutoOpt this has no real new advantages,
but for the drawing tool it is really useful. When you have to
fragments (or two atoms that you want to connect to make ring), you
can now easily connect them. The atom you are dragging is ignored and
doesn’t repulse the other atoms. As soon as you hit another atom, that
atom is fixed to make sure the atom stays at the mouse cursur.

Feedback is welcome.