``_assert_within_tol`` is often called with positional
 arguments that end up being filled into the signature,
 e.g. calls to ``_assert_within_tol(Y1, Y2, eps, verbose > 2)``
 evaluate to calls where `atol=eps` and `rtol=verbose > 2`.

 This PR converts calls to using keywords where needed. Further
 it modifies some tolerances to be stricter.

``_assert_within_tol`` provided a wrapper for ``assert_allclose``. This behavior
is already available from ``assert_allclose`` which is now used instead.

`atol` leads the tests to pass allowing the changes to the other
tests to be included in the 1.8 branch. This way some improvements
are included with the remaining test fine-tuning to be completed
after the branch.