Small fix for RangeSelection:

While parts of the code refer to `self.axis_index`, other parts refer
to `self._determine_axis()`. According to the doc, `self._determine_axis`
is only ever used, when `self._axis_index` is not set. In fact, the
property getter for `self.axis_index` handles this case, so `self.axis_index`
should be used in all cases.

It will only affect the behaviour if indeed `axis_index` is being assigned
to something different than the usual value, which will hardly every be.
Nonetheless, I guess we prefer correctness.