Tentative implementation of center-of-mass and of virtual atoms.

There is a generic ActionWithVirtualAtom class which can be used
as the base for Actions able to add new virtual atoms to the
system. Virtual atoms are stored in Atoms, can be referred to using
the label of the generating action. Automatically, they are
assigned an AtomNumber which exceed the real atom numbers
(e.g. if you have 108 real atoms, virtual atoms will have
indexes 108,109,...). They are automatically understood by
the parser provided one uses the parseAtomList() function.
Each instance of this class only generate a single virtual atom.
The nice thing is that, since lists of atoms can be dynamically changed
(e.g. by neighbor lists), dependencies are handled properly
and a virtual atom is only computed of the steps on which it is needed.

I also implemented a practical case, which is center-of-mass
(GenericCOM).