RF: make default slice axis adapt to time axis

Add tests for slice axis and time axis specified.

This is minor API breakage, because previous default behavior was:

    time_axis=-2, slice_axis default -> -2
    time_axis=0, slice_axis_default -> -2

Now:

    time_axis=-2, slice_axis default -> -1
    time_axis=0, slice_axis_default -> -1