Multi-language support for the openEHR terminology

Some time ago we discussed different issues with the openEHR terminology spec and XML representation: https://openehr.atlassian.net/wiki/display/spec/openEHR+Terminology

open issues are:

• how to improve multiple-language representation in the XML (XSD)

• if one or multiple files should be used

• discuss if we should add units to the openEHR terminology (or reference the UCUM ones)

Current XMLs:

• https://github.com/openEHR/specifications/blob/master/publishing/architecture/computable/terminology/terminology.xml

• https://github.com/openEHR/arch_ed-dotnet/blob/master/ArchetypeEditor/Terminology/terminology.xml

this is a related issue https://openehr.atlassian.net/browse/SPECPR-3