Interface to PLUMED

Note: Do not merge this yet, work in progress.

We add a macro to enable PLUMED support at compile time so that a separate patch does not need to be maintained.

PLUMED website: http://www.plumed-code.org/

Documentation for integration: http://plumed.github.io/doc-v2.0/developer-doc/html/_how_to_plumed_your_m_d.html