[firedrake] building a function space for slice vector fields

Dear all, A lot of what we do in the vertical slice model would be a lot easier if we could combine together our u-w components of our velocity field (stored as an RT field in the slice) with our v component (stored as a DG field in the slice) so that the basis functions are 3D vectors on a 2D x-z plane. In particular, then I could assemble the inverse of the combined u-w-v velocity mass matrix combined with Coriolis, and make things a lot more implicit in the Coriolis term, leading to bigger timesteps when we sent the Coriolis parameter to infinity (something we have to do in our test case unfortunately). Is this supported by current code, if not, how much work would it take for us to have this? all the best --cjc