Added GROMOS96 54A7 force field files.

Files are taken from ATB, which is hosted by Alan Mark, who
was an author on the GROMACS96 54A7 paper, which is cited in
forcefields.doc. So these files are effectively equivalent to
the reference implementation.

Fixes #773.

Change-Id: I800da3d3535a09cd5beb7f2b4d1b42422cc4329f