Implement Guzman-Neilan elements

[wence-]

These are a low-order, H1-conforming pair that discretise the stokes complex https://arxiv.org/abs/1710.08044

The pressure space is easy, but the velocity space is rather fiddly, it's P1 + normal components of face bubbles (these are a modification of the Bernardi-Raugel bubbles). This would also be a good test of extending the interface because the normal components will be piola-mapped, whereas the P1 part will be affine-mapped.