Use Trilinos Tpetra backend

[chrisrichardson]

I started working on something. Can you take a look @draenog?

[chrisrichardson]

Branch is chris/trilinos

[chrisrichardson]

TO DO:

• Add Trilinos (minimal required) to docker container
• Detect Trilinos properly with CMake (removing my hard-wired directories)
• Make Trilinos optional and only use it when available
• Use Tpetra/Muelu for the elastic problem too (dealing correctly with block size Tpetra::CrsBlockMatrix and local indices)
• Try to figure out how to use local indices for assembly (should be faster)