Add DofObject::var_to_vg_and_offset()
Add DofObject::var_to_vg_and_offset()

This will let us cache an O(n_variable_groups) operation and reuse it
over multiple dof number lookups.

roystgnr/libmesh,jwpeterson/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,giorgiobornia/libmesh,roystgnr/libmesh,pbauman/libmesh,giorgiobornia/libmesh,hrittich/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,hrittich/libmesh,pbauman/libmesh,hrittich/libmesh,90jrong/libmesh,libMesh/libmesh,hrittich/libmesh,libMesh/libmesh,hrittich/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,dschwen/libmesh,BalticPinguin/libmesh,pbauman/libmesh,dschwen/libmesh,libMesh/libmesh,pbauman/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,hrittich/libmesh,90jrong/libmesh,hrittich/libmesh,90jrong/libmesh,90jrong/libmesh,pbauman/libmesh,dschwen/libmesh,jwpeterson/libmesh,hrittich/libmesh,dschwen/libmesh,giorgiobornia/libmesh,hrittich/libmesh,roystgnr/libmesh,jwpeterson/libmesh,libMesh/libmesh,capitalaslash/libmesh,90jrong/libmesh,pbauman/libmesh,90jrong/libmesh,jwpeterson/libmesh,giorgiobornia/libmesh,libMesh/libmesh,libMesh/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,dschwen/libmesh,capitalaslash/libmesh,giorgiobornia/libmesh,90jrong/libmesh,jwpeterson/libmesh,90jrong/libmesh,giorgiobornia/libmesh,capitalaslash/libmesh,pbauman/libmesh,90jrong/libmesh,roystgnr/libmesh,libMesh/libmesh,roystgnr/libmesh,dschwen/libmesh,capitalaslash/libmesh,pbauman/libmesh,pbauman/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,capitalaslash/libmesh,giorgiobornia/libmesh,dschwen/libmesh