Fix atom masses in vanilla HybridTopologyFactory (#918)
Fix atom masses in vanilla HybridTopologyFactory (#918)

* take average of old/new masses for core atoms

* remove unused line

Co-authored-by: Iván Pulido <5392ba85d1ec0c6b3ed99eeb4e13a33f742e1838@protonmail.com>
Co-authored-by: Mike Henry <8f990534110b26309801dcd56dcee42ae40fee87@users.noreply.github.com>
choderalab/perses,choderalab/perses