Add method subPartitionType to entity

Summary

The PartitionType is a property of all entities, by the method entity.partitionType(). This is actually just an enum. Whenever you want to access the partition type of a sub-entity, you need to know the codimenion of that subentity at compiletime, i.e., you need to do entity.template subEntity<codim>(i).partitionType(). This is very complicated, especially if the codimension is not known at compiletime. Then a complicate compiletime switch need to be invoked in order to call this simple method. Maybe some grids have this information easier accessible?

This MR adds an interface method subPartitionType(i,codim) (similar to subIndex in the indexset) and provides a default implementation plus some specializations for a few classes where the partition type of sub-entities can be accessed more easily.