Units documentation

This documentation is based on the structure of
the StructuredAnnotations page under the aspect
"one of the trickier bits of the OMERO model".

The API for conversion is currently in flux so
the examples here will need to be kept in sync
with the code changes.
pity we don't just have a `p.getPhysicalSizeX().as(UnitsLength.NM)`, maybe someday!