Allow mapping of internal entities by dolfin/mesh/PeriodicBoundaryComputation.cpp

The periodic boundary computation is currently restricted to boundary entities only, but there are situations where one might want to map internal entities together.

The source code currently has a conditional statement on line 111 which could be removed:

// Consider boundary entities only const bool global_exterior_facet = (f->num_global_entities(tdim) == 1); if (global_exterior_facet) {