Revert "remove vestigial getCorrelatedNoise method for issue #527"

This reverts commit 1568efa4e990148803245631e6f1a164de06d2d1.
Change rng check in _BaseCorrelatedNoise to allow rng=None, which does work now.
(#526)
Mark im.getCorrelatedNoise() as discouraged and add it to CHANGELOG
(#526)