CombinedCalibratedRealInterval: add missing TODO
CombinedCalibratedRealInterval: fix method order
Avoid CalibratedSpace#unit and setUnit methods

These methods are redundant with the methods of CalibratedAxis, and
clutter the CalibratedSpace API. They will be deprecated imminently.
AbstractCalibratedSpace: simplify unit impls

Rather than throwing IllegalArgumentException (or worse: covering up the
problem by performing a no-op) when there are nulls, let's just let
NullPointerException happen since that is the purpose of NPEs: to be
thrown when something is null.
CalibratedSpace: deprecate unit/setUnit methods

These methods are redundant with the CalibratedAxis API. If we are going
to recapitulate such API for convenience, we should be consistent in
what we do. We do not reiterate other methods of CalibratedAxis, so why
do it solely for units?
CombinedCalibratedRealInterval: mark with FIXME

This class is currently broken as written and must be fixed ASAP.
CalibratedSpace: reinstate calibration API

For backwards compatibility, we should cut at least one release with
these methods still in place, but deprecated. Then afterward we can
remove the deprecated API. Otherwise, upgrading downstream code to new
versions of ImgLib2 is greatly complicated by the fact that API was
simply removed from release to release, with no deprecation period.

Keeping the methods available for one release allows downstream code to
upgrade to the new API in phases without breaking its build. Otherwise,
the entire upgrade must be done as a single monster commit, which is
difficult to code review and cannot later be bisected for debugging
purposes.