Fix for the pressure in MTTK with constraints + dispersion
Fix for pressure in MTTK - broken at some point in the
merge process. The MTTK+constraints code definitely needs to
be simplified for 5.0, it's too easy to break on code rearrangements
right now!
Testing with simple systems over long time shows that the pressure is
unbiased, while the PR pressure is off by a factor of 1/N (which is totally
fine for most realistic systems).
Fixes redmine bug #1061
Change-Id: Ie82cf8d6bdeb3e47db044ae34fc42c077ad73c75