Initial conversion code via ctors

As described on openmicroscopy/ome-documenation#1041,
this enables converting from most enums of one unit
to another.
Use target unit during conversion
Add a simple units tests for conversions
Add support for angstroms
Set unit on conversion
Python conversion ctors
Support Ice 3.4 enum "API"
Add getSymbol method to Java
Add getSymbol method to Python
Merge pull request #3227 from joshmoore/units-conversion

Conversion code for Java/Python