[DuneFunctionGFS] initialize EntitySet

When passing a dune-functions basis with a GridView to the DuneFunctionGFS, the GridView is wrapped in an EntitySet, but the latter is never updated. This required the user to create and setup the EntitySet externally and pass it to the dune-functions basis. This patch creates the EntitySet inside of the DuneFunctionsGFS and initializes it with all codims, circumventing the above problem. Note however, that the user cannot use a dune-functions basis with an EntitySet anymore. This fixes #74 (closed)