Cherrypick diagonal precon

[adler-j]

This basically cherrypicks the content of the diagonal preconditioning branch without the actual preconditioning, mostly minor fixups.

Was already reviewed.

[coveralls]

Coverage decreased (-0.04%) to 78.64% when pulling 179d185 on cherrypick_diagonal_precon into a8bb6fc on master.

[kohr-h]

"remove" in the commit message. Otherwise, I remember that I looked through that once.

[coveralls]

Coverage decreased (-0.04%) to 78.805% when pulling 9f61c47 on cherrypick_diagonal_precon into 1317d62 on master.