Slate with Jacobi

[sv2518]

This PR makes it possible to run a preconditioner based on Slate globally matrix-free but with jacobi preconditioner. In that case we ask to assemble the diagonal of an expression. Ideally this would have been part of the PR #2259 which we merged a month ago, since it's when I dropped the error message that "Diagonal + slate not supported" and introduced DiagonalTensors in Slate. The existing diagonal infrastructure only had to be adapted to give the diagonal tensor back as a vector. Before this PR we were not able to run a solve in a Slate based preconditioner with Jacobi and 'mat_type': 'matfree' in its parameters

  'hybridization': {'ksp_type': 'cg',
                    'pc_type': 'jacobi',
                    'mat_type': 'matfree'}}