This is a copy from the branch CMap_no_dart-gdamiand by keeping only the part with dart_handle.
The modifications are the ones given in the small feature Combinatorial map dart attribute.
* clarify all the tests of group/ungroup cases for non void attributes
* create specialized functors allowing to consider all the different cases
(group two edges along their two extremities, only the first or the second...)
* make many cleanup to move functors in different files, and to comment them
* add namespace before global functions
Still to do: sewable for dim>3 (build the isomorphism); try to improve onsplit
test by testing only one out of two modified darts; add a parameter to
operations to do not test group/ungroup; do a functor which updates the
attributes of all the map (avoid do make many small tests, and do only one
big test once).