ISO 80000-8: Decibel `dB` and Bel `B` are in the database but not supported in UnitsML

[ronaldtse]

From @anermina #36.

It should also be checked why dB and B are currently not supported even though they are in the database.

These are defined in ISO 80000-8.

Originally from #36

[ronaldtse]

@anermina this issue has been resolved via #102 . Both B and dB are now supported and have the correct quantities. Closing.