Improve unit interface

[jesper-friis]

The dataset.py module has basic support for units via the functions dimensional_string(), get_unit_symbol() and get_unit_iri() functions.

These functions imports EMMO, but currently not the modules non-default modules like prefixedunits, etc.

Needed features

• Add optional support for accessing prefixedunits, etc (should be the default?)
• Add support for accessing units using QUDT and OM IRIs

Some of these features may be moved to tripper.