Hybridisation using firedrake.SCPC

[JHopeCollins]

Pulling out the reimplementation of hybridizationPC using SCPC from #152

This implementation requires a form_mass and form_function from the appctx, but will work with the vector implementation of complex-proxy as well as with >2 fields, whereas HybridizationPC can do any(?) form but is restricted to plain HDiv x L2 and only 2 fields.

Required:

• Build form with form_mass and form_function
• Allow the HDiv space to be at any index
• Combine the complex-proxy implementation with the real implementation
• Test for checking that the PC solves the real LSWE in one iteration
• Test for checking that the PC solves the complex-proxy LSWE in one iteration

[JHopeCollins]

Additions (probably for later PRs as needed):

• Deal with boundary conditions
• Deal with extruded meshes with tensor product elements
• Deal with nullspace
• Add a view method