MAINT: create ensure_symmetric utility function to check matrix symmetry
rename ensure_symmetric -> test_symmetric
TST: add test of check_symmetric
DOC: fix docstring of check_symmetric
add more informative doc & error message in utils.check_symmetric
Merge pull request #4024 from jakevdp/ensure_symmetric

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