Generalize IForceProvider

Generalize IForceProvider

- Remove knowledge of individual modules from t_forcerec, and hide the
  number of modules, and whether they contribute to a virial or not,
  behind a generic interface.
- Improve hard-to-understand initialization of separate f_novirsum,
  resolving some TODOs.
- Add some parameters that probably make sense for other modules beyond
  the electric field one.

This also makes the requirement for all modules to have the same
parameters for the calculateForces() call explicit.

Change-Id: I4952515c4b707ba458fd267565fd000532ec281e