Remove redundant attributes in SdoTermSource

[MatthiasWiesmann]

Removed the id, uri and enum fields of the SdoTermSource objects as these are not needed.
The id and uri fields are copies of what is present in the relevant fields of the termdesc field.
The enum field was used to cache a value that is only used once, in the constructor.
Change the isEnumeration to be a class method that takes the term_id as parameter, and marked it protected.