All handle filtering of units via marker interface
gateway: helper to unwrap units

As with the pojo objects, the current helper
is just a workaround. Either the clients will
need to work with the unit types themselves
or pass around a tuple like `(double, unit)`.

fixup: gatweay (rword)
Use ome.model.enums to convert symbols
Move UnitsFactory methods to omero.model classes

This modifies much of the API which was created on this
branch for dealing with new units. Important to note
are:

 * Use of the XML default strings is **discouraged**
 * UnitsFactory now contains the defaults **per field**
 * Otherwise, most of what can be done with UnitsFactory
   can be done with the concrete classes themselves.

fixup: convert to new LenghtI etc
Set strokeWidth lengths in OmeroPy
Various minor javadoc/formatting fixes
Fix pixel/reference frame entries
Fix return type in IceMapper
Fix use of lower-case symbol enums