p3m: Make sure p3m isn't startet with nonmetallic epsilon and noncubic box. This is not implemented and would result in wront results.
Corrected faulty mesh density calculation in P3M tuning.

Conflicts:

	src/p3m.c
Fixed wrong starting mesh calculation in p3m tuning.
Merge branch '3.1' of github.com:espressomd/espresso into 3.1