Try an unordered_set to speed up count() calls
Try an unordered_set to speed up count() calls

This also avoids the lookup in Mesh::elem_ptr

Hopefully this adds to the CheckpointIO optimization in #1859

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