[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.
No, the latter approach doesn't work for LIL. Its `data` is an array of lists.