Change API of constraints/projectors and the C++ backend.

- Uniformize pyfaust and matfaust ConstraintMat ctors.
- Add functors splin(), supp()... to simplify use of projectors/proxs.
- Rely on C++ backend Faust::prox_const() (added by this commit) instead of copy the matrix in wrapper code.
- Manage the option to normalize (fro. norm) any projection image afterward.
- Add also a 'pos' flag to allow "filtering out" the negative part of the matrix projected.
(and other minor changes)