[WIP] Attempt to fix anisotropic Monte Carlo barostat tests

This is very much a work in progress.

There were several flaws with the Einstein crystal test (see #348).

I was able to correct the weak spring constants, but the sticking point now is that the grid of crystal centers was not scaled when the barostat scaled particle positions.  I tried to rectify this by adding a grid of fixed noninteracting particles and a grid of mobile noninteracting particles tethered to the fixed grid of particles by a `CustomBondForce`.