unique_rows does not support tolerance

[VolkerH]

While the tests for unique_rows also test float arrays, unique_rows does not support a tol argument such as unique. I tried to fix this by replacing the call to np.unique in https://github.com/nschloe/npx/blob/main/src/npx/_unique.py#L53 to a call to your unique that supports a tol argument but that simple fix did not do the trick unfortunately (led to some ufunc dtype incompatibility exception) It would be nice if unique_rows also supported a tol argument.