Have `get_units` always return a `Unit` (and not a `Quantity`). Closes #329
Convert scalar quantities to strings in a more similar way to quantity arrays, e.g. use "2. mV" instead of "2.0 mV" and use scientific notation when necessary. Also make a better choice for the best unit. Closes #322
Merge pull request #333 from brian-team/unit_fixes

Unit fixes