Release Name: 5.1

Change Log
This following bugs were fixed in OpenMM 5.1:

1833: RPMDIntegrator should apply periodic boundary conditions based
on all copies
1845: VirtualSites don't work with RPMD
1848: Kinetic energy is incorrect for systems with constraints
1852: GRO files with velocities are read incorrectly
1861: Exception in step() causes a segfault for Python