Fix inconsistencies in DiscreteDerhamMultipatch

[yguclu]

The class DiscreteDerhamMultipatch is a subclass of DiscreteDerham (single patch by default), but it does not implement all the basic methods. In particular, the derivatives method which returns the differential operators is not implemented.

-> Make sure that all the functions of DiscreteDerham are well implemented for DiscreteDerhamMultipatch.
In particular, for a multipatch dR sequence the derivatives should return the broken_derivatives (as operators, matrices), which should be understood as the broken ones