Implement Bernardi-Raugel elements

[David]

Dear all,

I'd like to propose supporting Bernardi-Raugel elements (also called "reduced P2/P0") in FEniCs. BR-elements are a inf-sup stable elements pair for the Stokes problem. The velocity space (for first order) is P1, enriched with face bubbles. The pressure space is discontinuous P0.

Refs: John (2016), Remark 3.138, Bernardi & Raugel (1985), Carstensen (2015)

Thanks in advance!

David