Remove equivalence between custom strain unit and 'dimensionless_unscaled'
This PR improves the handling of the 'strain' unit by removing the equivalence with astropy's dimensionless_unscaled. This means that it won't be silently dropped during unit operations.
Before:
>>> import gwpy.detector # register the 'strain' unit
>>> from astropy import units
>>> units.Unit('strain') * units.Unit('second')
Unit("s")
After:
>>> import gwpy.detector
>>> from astropy import units
>>> units.Unit('strain') * units.Unit('second')
Unit("s strain")