Initial conversion code via ctors

As described on openmicroscopy/ome-documenation#1041,
this enables converting from most enums of one unit
to another.

/cc @dominikl @will-moore @jburel 

see: https://github.com/openmicroscopy/ome-documentation/pull/1041
Wrong unit, if a conversion has been applied, unit has to be set to target unit.