Added load monitoring to Ewald

Load monitoring was previously only applied to force calculations due
to Potentials, but Ewald if applied is usually the performance
bottleneck. Therefore, it is crucial to monitor the cpu load balance in
MPI also for Ewald, in force, energy and electronegativity calculations.

Also, some documentation updates are included.
Added sign check to charge_pair_abs potential

The potential contains a square root of a function that may be
negative, which can lead to NaN. If negative values are encountered,
the potential is given a zero value to avoid problems.