Merge pull request #1458 from mhvk/units-sqrt

sqrt in units -> should return unit**0.5, not unit**-2
Define percent as Unit(0.01), and ensure its representation is '%'
Ensure % is recognized in generic parser, as well as in compose()
Correction to not mess with the registry, remove testing statement, PEP8
Roundtrip for generic/CDS, exceptions for scaled dimensionless for vounit
Use is_effectively_unity to test for scale=0.01
Correct tests that changed now that '0.01' * Unit gives a unit
Fix bug where FITS and VOUnit unit parsers were not using the correct implementation of _parse_unit
Add tests that % is correctly interpreted by generic and CDS