Fixed missing import of Reaction class in rmgpy.pdep.network.

Ironically this error was only occurring when about to raise a different
exception.
Changes to help run the RS method for a 2D master equation.

The new version iterates over J when choosing the active-space matrix
bandwidth. Several subscript errors are also fixed (e.g. indexing by one
subscript when two are required).
Do not raise exception within `isMoleculeFobidden` for program speed-up

Uses slightly more restrictive initialMap.