Add dex, dB units; skip tests for mag,dB.decompose() for CDS, VOUNIT
since these do not know dex
Avoid clash between dB and decibyte=dbyte=dB
Add equivalency for logarithmic -> dimensionless, plus tests
Allow complex scale -- for (mag**0.5).decompose()
Fix coverage test
Prepare for functional units by allowing subclasses to get priority
Introduce functional and logarithmic units; test cases for latter
Name change forward/backward->to/from_physica; more docstrings and comments
Changes following rebase: ones_like, no CompositUnit._simplify()
Changes following rebase to unit/low-hanging-fruit
Small correction following rebase to units-dex-dB