use physical functions in multipatch projector interface

[campospinto]

We should use physical functions in the multipatch projector interface.

Quoting one comment by Yaman in PR #320:

On the one hand, the method projectors of the class DiscreteDerham (which works on single-patch domains) provides commuting projectors for the physical fields, as the pull-back is applied internally. See here.

On the other hand, the method projectors of the class DiscreteDerhamMultipatch provides commuting projectors on the logical fields, as no pull-back operation is performed internally. See here.

For reference, those methods use the internal classes Projector_H1 and Multipatch_Projector_H1, which work on logical fields. These classes are in charge of performing (patch-wise) global interpolation in a parallel setting.

We should also add module/class/function docstrings in NumPyDoc format to the file psydac/feec/multipatch/utils_conga_2d.py