Remove assertion from UnstructuredMesh::copy_nodes_and_elements()
Remove assertion from UnstructuredMesh::copy_nodes_and_elements()

It should be possible to call this_mesh.copy_nodes_and_elements(other_mesh)
even when other_mesh is partitioned onto more procs. To allow this, we simply
"wrap around" processor ids to fit into our current range. This could
produce an arbitrarily bad partitioning, but this should be fixed up
by the prepare_for_use() call which comes later in this function.

Refs #1512.

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