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
Another attempt to ensure __repr__ works on both python2 and 3, including for unicode units from CDS
Ensure u.mag ** 0.5 works
Add comments for why is_effectively_unity may have to deal with complex scale
Move import cmath to top
remove unnecessary import in core, two small PEP8 changes
Skip mag.decompose() test for CDS and OGIP, as they cannot handle dex
Merge pull request #1628 from mhvk/units-dex-dB

Add dex, dB units