Support differing masses in .HDF5 input

[mortele]

Allow masses of different species (or even individual particles) to differ by supporting a masses array of floats in the input .HDF5 file.

[mortele]

Non-exhaustive checklist of changes needed:

• Change the scaling of the center of mass momenta remover in main.cancel_com_momentum(...), can no longer apply the average across all particles, need to scale with the masses.
• Change the scaling in the same way for the center of mass momenta removal internal in the thermostat. If the cleaning is done per group, this is only necessary if masses can vary within the same species.