Gromacs-like replica exchange is there!

I finally upload the GREX stuff. It seems to work, but I did not really
debug it (there might be wrong units or so). I also provide a patch
for gromacs 4.5.3. Interface is still a bit tricky, and probably it
will be necessary to adapt it a bit as we try to port it to other codes.

The overall flow is:
* at steps corresponding to attempts, positions/total energy are
  saved on a temporary buffer when normal plumed is called.
* when repl_ex is called in gromacs (which, in the case of gromacs
  is after moving atoms, so with different positions) and
  the partner replica is decided by gromacs, a fake exchange of all coordinates
  is done inside plumed and a new calculation is also done.
  Notice that since presently all "analysis" tools are acting in the
  calculate() method, they are called once more at this stage. This
  should be fixed.
* Then, change in local bias potential and foreign bias potential
  are collected by gromacs and added to the acceptance.

Notice that with this approach the plumed.dat files used for different
replicas could be completely different (but for UNITS! this probably
can be fixed, even if it seems a bit stupid to me), and, a part from
trivial errors which could be there, the acceptance
is evaluated consistently