Utilize unit conversions in imglib2-meta

Now that there is a unit conversion facility in imglib2-meta we should utilize it as possible. For instance we need to finish writing CombinedCalibratedRealInterval such that you can define axes and modify underlying axes by scaling utilizing unit conversion code. And also unit code can be used for pulling data out of interval.

Migrated-From: http://trac.imagej.net/ticket/2004

Start by locating the unit conversion facility in imglib2-meta and the CombinedCalibratedRealInterval class mentioned in the issue. Trace how axes are defined and how interval data is retrieved; no test or file path is named, so identify the relevant tests before changing anything. Done means axes and underlying-axis scaling use unit conversion, and interval data can be pulled out through the unit-aware path.

Written by the indexing model from the issue text.