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