Add const get_periodic_boundaries API in DofMap
Add const get_periodic_boundaries API in DofMap

I have a `const DofMap *` data member in a class and I want to retrieve
its periodic boundaries for setting periodic boundaries in a ghosting
functor I am initializing, but I currently can't do it because we just
have the non-const API

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