[MRG+1] MAINT: create ensure_symmetric utility function to check matrix symmetry

(first commit of this is from PR #4021)

This PR creates a utility function called ``ensure_symmetric`` which checks whether a matrix is symmetric. This also fixes a (possible?) bug in the validation check for ``spectral_embedding``: it compares the absolute difference of the matrix with its transpose.
Now that I'm reading it right, I don't think there's any problem. Sorry for all the noise.

I would consider adding in a `tocsr()` before `.data` so that it's future proof.