Parrinello-Rahman barostat for modular simulator (leap-frog)

Parrinello-Rahman barostat for modular simulator (leap-frog)

This introduces the Parrinello-Rahman barostat for the leap-frog
version of the modular simulator. Mirroring the implementation in
the legacy do_md, this does not support md-vv. Implementing the
pressure coupling requires the virial calculation to be synchronized
with the pressure coupling, so this is included in the energy
signaller.

As the new integrator framework is not used by default, this
patch adds equivalence tests for md Parrinello-Rahman pressure
coupling.

Change-Id: Iaadd78a3da1ee2b04978ab7c7a7caecbb03181e2