QM/MM coupling

Hi all - Axel has created a new fix qmmm that allows

LAMMPS to be coupled to Quanum ESPRESSO to

do QM/MM calculations. This is a first step, but Axel
is hoping others will join the effort to produce more
complete couplings to QE and other quantum codes.

See the doc page for fix qmmm and the lib/qmmm/README

file to get started.