Make `DiscreteBilinearForm` in `psydac.api.fem_bilinear_form` work on 3D multi-patch domains

[yguclu]

The sum factorization algorithm introduced in PR #448 only works for the assembly of bilinear forms in 3D single-patch domains. When discretizing over a 3D multi-patch domain, the constructor of the new DiscreteBilinearForm fails in multiple places. The constructor logic is designed to handle multi-patch domains, but it contains some bugs which should be fixed. This problem has not been caught so far because of limitations in our 3D unit tests.