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 were updated as needed to reflect the removed AxisTypes and new
DefaultAxisType class.
AxisType: Deprecated isXY and isSpatial

Fundamentally, AxisTypes should not be defined by their label. The label
uniquely identifies and distinguishes axes and these methods are tied to
a 5D model, where X and Y are significant and XYZ are spatial.

To move to an N-D model, what really matters is the order of the axes
and the number of planar axes. Thus isXY and isSpatial would become
unnecessary.

These methods are thus deprecated and when conversion of consuming
(SCIFIO) code to an N-D model is complete, and propagated downstream
(e.g. to IJ2), these methods can be removed.
Merge pull request #31 from imagej/axis_grouping

Refactored Axes from a sort of hybrid enum/static utility class to a pure static utility class. The enum aspect was unnecessary and not easy to leverage, and it was confusing to code against both Axes and AxisTypes.

Axes is now purely a static utility method for creating AxisTypes. It has a fixed set of standard axes (XYZCT) and methods (#get) to create axes with custom labels, or unknown (#unknown()) axes.

Consuming software requiring an additional set of standard axis labels should create new static utility methods that make calls into Axes.get to provide their custom AxisTypes. This will ensure one central location (the Axes class) for all AxisTypes.

Also, as we are moving to a more flexible Axes model, the AxisType isXY and isSpatial methods were marked deprecated. These methods imply a very specific use of axes, and attach metadata unnecessarily to the AxisType class.

When all consumers (SCIFIO and IJ2 specifically) are converted to N-D models these methods can be removed.