Modified output files for writing atomic forces, and fix to path-integral forces
in ab initio path integrals.
Merge pull request #48 from TuckermanGroup/force_output

Writing of atomic forces and AIPIMD force fix
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Minor
Minor - syntax
Style cleanup
Initial implementation of PLUMED interface

Basic features seem to be working fine.
Needs testing.
Needs extending to
- propagators other than NVT
- parallel
- PIMD
Fix offset errors
Add PLUMED to the NVT RESPA propagator.
Add PLUMED build settings on OS X
Fix timestep with RESPA
PLUMED on the PIMD centroid
Remove Linux-specific settings.

Set DYLD_LIBRARY_PATH instead.
Better build settings
Remove comment line
Parallel, does not quite work