net.imglib2.meta: clean up style of all classes

This was done using Eclipse's "Clean Up" feature according to the ImageJ
style template. The ImgLib2 style is quite different though so at some
point we may need to reformat all ImgLib2 classes according to an
ImgLib2 style template (if/when such a thing is ever created).
Add missing javadoc for more unit tests
Improve unit test method names and add comments
DefaultCalibratedSpaceTest: test for unknown axes

The previous check only verified that the axes had a custom type. It is
more appropriate to verify that the label is "Unknown" as expected.
DefaultCalibratedRealInterval: use default axes

This logic is analogous to DefaultCalibratedSpace. If one of these
default implementations assigns IdentityAxis axes, they both should...
or else both should be null, for consistency.
LinearAxis: split interface and implementation

The reason is that we have a special LinearSpace, and in several places
we check whether an axis instanceof LinearAxis. So if LinearAxis is not
an interface we hamstring extensibility here. Perhaps ideally all the
various Axis implementations should be interfaces, but it is less
critical than for LinearAxis at this juncture.

There is also an issue with a circular package dependency between
net.imglib2.meta and net.imglib2.meta.axis:

- Both CalibratedSpace and CalibratedAxis (a class dependency of
  CalibratedSpace) are located in net.imglib2.meta.
- LinearSpace is also in net.imglib2.meta, but LinearAxis (a class
  dependency of LinearSpace) is located in net.imglib2.meta.axis.

So the design is clearly inconsistent, package-wise. However, the
consequences to consuming code are minimal from a practical perspective,
so rather than repackage a whole bunch of classes to avoid any hint of
circular package dependencies, we leave things as they are for now.
Add notes about transient LinearAxis dependency
IntervalUtils: remove unused methods

These methods are not being used anywhere in either ImgLib2 or ImageJ2.
We can add them back if we find ourselves reimplementing this logic, but
I think it is only limitedly useful to boil down a RealInterval's min
and max to a single extent (max - min), calibrated or not.
IntervalUtilsTest: use more descriptive method name