`openmmtools.testsystems.DoubleWellDimer_WCAFluid` perhaps uses inappropriate periodic box size

[dominicrufa]

https://github.com/choderalab/openmmtools/blob/7a9c61cea5c657e333f433dabbd7c87624f8227f/openmmtools/testsystems.py#L2334-L2337.

i think the appropriate calculation of box length should be

volume = (nparticles * sigma**3 / density)
length = (volume.value_in_unit_system(unit.md_unit_system))**(1./3.) * unit.nanometers