Extend is_nonneg() of diag_mat to check if arg is psd [issue 1213]

[phschiele]

This PR adds @SteveDiamond 's proposed improvement for the is_nonneg() check in the diag_mat class, as per discussion in #1213. Also, the tests are extended to cover this case.

Contributing guideline checks:

• no regression change
• no relevant change in benchmark timings (25.33s pre-change vs. 25.05s after the change)
• flake8 passing

[SteveDiamond]

LGTM! Thanks for fixing this!