New VDW kernel flavour for LJ-PME and updated group kernels

Group-scheme LJ-PME now calculates the same properties as Verlet-scheme

The previous group kernels implemented the non-bonded interactions
as described in J. Chem. Theory Comput., 2013, 9 (8), pp 3527–3537,
These are now updated to subtract the grid contribution in real
space, which corrects for any difference in the reciprocal-space
part from the use of approximate LJ parameters.

Group kernels + LJ-PME now supports modifiers

Added a new interaction type "LJEwald" which is used in the
group scheme for simulations with LJ-PME.

Change-Id: I4c9394381bf813082afe1fdbc168659de2412da6