Support for Bels
While the units library has support for logarithmic scales, it does not support the Bel (as in decibel, dB). Bels are interesting because they are not units themselves, but rather a unit "template" that can be applied to any unit.
See also #3 as there may be common functionality required for both. Some tricky things to consider are how reference points could be distinguished; for example decibels of sound pressure levels (SPL) are often recorded at the distance of the measurement from the source of the sound (dbA @ 1m). Should we support those cases and, if so, how?