Units for charges and masses

[GiovanniBussi]

Currently, units for charges and masses are not modifiable in plumed. This lead to the issue #165 , related to the assumption that charges are in units of e in all MD codes.

Mass units are currently irrelevant in plumed, as masses are only used in computing centers of mass. Charge units would affect the calculation of DHENERGY and DIPOLE. Both units are visible when using DUMPMASSCHARGE.

I would like to make the treatment of charges and masses uniform with respect to the one of positions, namely:

• Allow user to specify units in the UNITS action
• Allow the MD code to specify used units (this would allow for an easy solution of Wrong charges passed from AMBER patch #165
• Allow driver to accept --charge-units and --mass-units to know how charges and masses are defined in the file passed with --mcfile