Use the elem_num_map when reading elemental variable values.

We came across this bug while reading elemental variable fields from an Exodus file generated by Cubit 14, which uses a non-trivial element numbering map by default.  The code for reading elemental variable values was not taking this map into account.  I want to make sure this still passes the moosebuild CI, although I don't believe we exercise this particular aspect because we don't have any tests with Cubit 14 files.