d dimensions expand/contract together
  uniformly using pressure components averaged over those 2 dimensions
  as the driving force, the non-specified dimension will expand/contract
  independently using its pressure component as the driving force
for anisotropic, all 3 dimensions expand/contract independently using
  individual pressure components as the 3 driving forces
in all cases, the simulation box stays rectilinear (not Parinello-Rahman)
for dimensions coupled together, their specified P values shoul