Add DofObject::get/set_extra_datum
Add DofObject::get/set_extra_datum

I want to store non-integer data on nodes, and our only dynamically
reallocatable structure here is integer-only, so let's encapsulate the
conversions as much as possible.

BalticPinguin/libmesh,hrittich/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,hrittich/libmesh,dschwen/libmesh,roystgnr/libmesh,hrittich/libmesh,hrittich/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,roystgnr/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,hrittich/libmesh,capitalaslash/libmesh,roystgnr/libmesh,roystgnr/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,hrittich/libmesh,capitalaslash/libmesh,libMesh/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,hrittich/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,capitalaslash/libmesh,roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,capitalaslash/libmesh,hrittich/libmesh,libMesh/libmesh,capitalaslash/libmesh,hrittich/libmesh,libMesh/libmesh