Use cubic crystal to test NPT integrator
Use cubic crystal to test NPT integrator

-because of the finite shear elasticity of the solid, this avoids
large tilting angles in the triclinic test
-use GPU classes where appropriate to speed up GPU unit test

refs #1534

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue