Catch LennardJonesGenerator made NB functions.
Catch LennardJonesGenerator made NB functions.

Correctly raise a NYI error when the nonbonded force's LJ terms are
handled by a tabulated potential generated by the OpenMM forcefield.py's
LennardJonesGenerator function (Such as CHARMM36). This will prevent
the alchemical factory from continuing and act like everything is
correct when it isn't.

Temporary catch until #510 can be fixed.

Fix the case there is a CustomNB force that doesn't have the tabulated
expression.

choderalab/openmmtools,choderalab/openmmtools