Axes: refactor to static utilty class

Having Axes as an enum, even an extensible enum, was not overly helpful.
Because ultimately most operations are on AxisType itself, you can only
take advantage of the Axes enum if you type everything as an Axes, which
then excludes any other AxisType implementation.

Instead, it makes more sense to just use AxisType and turn Axes into a
static utility class that defines a set of AxisTypes. This allows us to
reuse the extensibility logic in that consumers (other static utility
classes) can now invoke the get(label) method of Axes to define new
AxisType constants, which are then stored centrally.

Axes now defines the core XYZCT axes, with the more "specialty" axes
moved to SCIFIO.
Tests: restrict axes to XYZCT

As the non-core Axes were moved to SCIFIO, their (arbitrary) use in the
various Typed and Calibrated axis/space/interval tests has been changed
to XYZCT references.