Simplify ExodusII_IO_Helper::get_nodeset_data_indices()
Simplify ExodusII_IO_Helper::get_nodeset_data_indices()

We now return the nodeset data array indices for all nodesets defined
on the mesh, regardless of whether or not they have nodeset variables
defined on them. This is a superset of the original behavior, so it
is not likely to break any existing code.

Refs #3213

libMesh/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh