Merge branch 'feature/add-subentities-to-all-codims' into 'master'

add method Entity::subEntities to all codimensions

For polyhedral grids, it is not clear how many vertices a face (i.e., a general polygon) has. For elements, this information is provided by the (redundant) method Entity::subEntities. This patch add this usually redundant method to entities of all codimensions.

See merge request !103 (merged)