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.
HyperSliceImgPlusTest: fix javadoc
ImgPlus: fix javadoc and section headers
ImgPlus: deprecate getImg method

This change is in preparation for ImgPlus's refactoring. It will
probably become an interface, and not necessarily always be a wrapper,
so the getImg() method does not always make sense anymore.
ImgPlus: call AxisTypes "types" rather than "axes"

This is in preparation for the ImgPlus refactoring that will switch from
AxisType to Axis (specifically TypedUnitAxis).
Eliminate unnecessary usages of ImgPlus#getImg()
Deprecate CalibratedSpace hierarchy

It will be replaced with a new CalibratedSpace that extends TypedSpace.

The existing CalibratedSpace classes have been renamed to
OldCalibratedSpace.
Add CalibratedSpace extension of TypedSpace

This space attaches a unit and calibration to each of its dimensions.
Utils: Use CalibratedSpace in getSpatialCalibration
InteractiveImageViewerExample: use new CalibratedSpace
RandomAccessible: fix up javadoc
FinalInterval: tweak javadoc
ImgPlus: use new CalibratedSpace

This commit also makes the necessary changes throughout the codebase to
accommodate this change in the type hierarchy.
Axes: minor fixes and refactoring

The following changes are part of this commit:
  * Add ability to construct custom spatial axis types
  * Rename CustomAxisType to simpler CustomType (makes calling
    "new Axes.CustomType" directly less redundant-looking)
  * Tweaked javadoc
AxisType: tweak javadoc
TypedSpace: each AxisType can only be used once

We have generally assumed that for a single TypedSpace, there cannot be
multiple axes with the same type. Maybe later we can add argument
checking for it, but for now, let's just mention it in the javadoc.