Add missing EntitySet functions and declarations to be an EntitySet concept
The entity set behaves as a grid view but with a different set of indices. The index set implementation is missing some features to pass as a regular index set in a concept check. This MR fixes those issues. See https://gitlab.dune-project.org/santiago.ospina/dune-concepts/-/blob/e9fd314c6f2c25d27dfc28ba52891a6a1b919660/dune/concepts/grid/indexset.hh
Edited by Santiago Ospina De Los Ríos