Allow scaled dimensionless units for CDS format
Touch-ups: --- is dimensionless; unicode MULTIPLICATION SIGN can be used
Merge pull request #1465 from mhvk/units-cds-scaled-dimensionless

Allow scaled dimensionless units for CDS format
Minor typo fix in documentation; the module is 'angles' not 'angle'.
Slightly alter description of `mag` unit

Previously it was "stellar magnitude", which isn't right because magnitudes are regular used for things that aren't stars.
Add a system whereby collections of units can be enabled after the fact.
Remove imperial units from examples
Fix documentation typo
Add astropy.units.imperial.enable