Bugfix in setting of Lennard-Jones epsilon value.
Bugfix in setting of Lennard-Jones epsilon value.

The previous implementation - designed to accept either temperature or
energy units - did not handle None in a reasonable way.

nyee/RMG-Py,nyee/RMG-Py,comocheng/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,faribas/RMG-Py,comocheng/RMG-Py,KEHANG/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,faribas/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,KEHANG/RMG-Py