Distinguish between non-coforming and discontinuous

[chr-engwer]

Elements like Crouzeix–Raviart or Ranacher-Turek (rotated bi-linear) are not fully discontinuous, but ansure continuity in the center of the face.

I suggest to distinguish between these two variants in the categorization.

[mscroggs]

See also third bullet point of https://github.com/DefElement/DefElement/issues/364