Split net.imglib2.meta to new imglib2-meta module

The rationale is that we are planning to expand the metadata class
hierarchies, adding new Axis subclasses and new interfaces. Some of this
work will introduce at least one new dependency on a units management
system (probably UOMo). To avoid inflicting that dependency on projects
that do not care about such metadata, we separate things out.
Axes: remove unused static isXY method

You can accomplish the same thing more directly by calling isXY on the
AxisType itself.
Axes: remove built-in UNKNOWN type

Instead, the new static method Axes.unknown() returns an axis of unknown
type, which is always a new object.

The problem was that multiple unknown axes would test as the same type,
when in reality two unknown axes are just that -- unknown -- and should
test as different types.
Add abstract superclass for AnnotatedSpace

This provides a convenient extension point for subclasses.
Add TypedSpace extension of AnnotatedSpace

This space attaches an AxisType to each of its dimensional axes.
Add UnitSpace extension of AnnotatedSpace

This space attaches a unit and calibration to each of its dimensions.
Add TypedUnitSpace extension of AnnotatedSpace

This extension is a combination of TypedSpace and UnitSpace.
WIP: Utils.getSpatialCalibration(Metadata) -> TypedUnitSpace