remove assert for frequency=0; it is valid for unbound Poisson-type systems
remove assert for frequency=0; it is valid for unbound Poisson-type systems

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