Create Python functions for querying the database for DomainToolUnit mappings

[adamnagel]

The database now includes concepts for capturing Domain Tool Unit mappings, going to and from QUDT. Create some library functions for finding domain tool units from QUDT units, and vice versa.

This may take the form of finding "Domain Tool Unit Synonyms" for given QUDT or (advanced) non-QUDT units.

Some function ideas:

• GetDomainToolUnits(qudt_unit_uri): returns a list of URIs of domain tool units that link to the QUDT unit.
• GetEquivalentUnits(any_unit_uri): returns a list of URIs of QUDT and non-QUDT units that are equivalent with the given one.
• GetDomainUnit(qudt_unit_uri,domain_class_uri): returns a list of URIs of equivalent units that are members of that domain class.

[adamnagel]

Deprecated, we should re-do the Python lib to match the new C# lib.