Added interface to Gromacs-like Replica EXchange

I am starting to merge my GREX branch, which should be
general enough to be used in any REX framework where
coordinates are swapped (a-la-gromacs). Since GREX
significantly modifies the MD-PLUMED interface through
the addition of several new cmd(), I tried to encapsulate
it as a separate objects with a cmd()-like interface.
Commands from MD starting with GREX are automatically
forwarded to that object. E.g.
plumed_cmd(plumed,"GREX init",NULL);
or so should initialize the GREX object, etc.

Up to this point I just upload the interface, later
on I will add the real GREX object, for which I still
have to do some cleaning.