thomasgibson/scpc: Firedrake preconditioners for static condensation and hybridization

Version used in the Springer brief: "Automating the finite element method for geophysical flows using Firedrake." This release is specifically created to document the version of Firedrake used in a particular set of experiments. Please do not cite this as a general source for Firedrake or any of its dependencies. Instead, refer to http://www.firedrakeproject.org/publications.htmlVersion used in the Springer brief: "Automating the finite element method for geophysical flows using Firedrake." This release is specifically created to document the version of Firedrake used in a particular set of experiments. Please do not cite this as a general source for Firedrake or any of its dependencies. Instead, refer to http://www.firedrakeproject.org/publications.htmlfiredrake_03202019_v