Add global registry of representations and default names and units
Add global registry of representations and default names and units

Each representation is automatically registered when the class
is defined.  That provides a mapping between the alias name and
the class.

Each representation now defines the default names (e.g. ra, dec,
distance) and units (e.g. u.deg, u.deg, None) for that
representation.  These are only used in frames but here seems like
the most logical place to store this information.  Otherwise there
would need to be a secondary data structure, and authors of
new representations would need to have a way to add their frame
to that secondary structure.

kelle/astropy,bsipocz/astropy,mhvk/astropy,larrybradley/astropy,joergdietrich/astropy,dhomeier/astropy,saimn/astropy,astropy/astropy,AustereCuriosity/astropy,dhomeier/astropy,tbabej/astropy,mhvk/astropy,AustereCuriosity/astropy,MSeifert04/astropy,pllim/astropy,astropy/astropy,pllim/astropy,dhomeier/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,joergdietrich/astropy,dhomeier/astropy,AustereCuriosity/astropy,funbaker/astropy,saimn/astropy,mhvk/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,saimn/astropy,astropy/astropy,pllim/astropy,tbabej/astropy,lpsinger/astropy,stargaser/astropy,MSeifert04/astropy,joergdietrich/astropy,kelle/astropy,DougBurke/astropy,lpsinger/astropy,bsipocz/astropy,MSeifert04/astropy,bsipocz/astropy,lpsinger/astropy,tbabej/astropy,stargaser/astropy,funbaker/astropy,AustereCuriosity/astropy,saimn/astropy,larrybradley/astropy,mhvk/astropy,MSeifert04/astropy,bsipocz/astropy,tbabej/astropy,DougBurke/astropy,mhvk/astropy,DougBurke/astropy,funbaker/astropy,stargaser/astropy,stargaser/astropy,pllim/astropy,joergdietrich/astropy,kelle/astropy,saimn/astropy,kelle/astropy,kelle/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,StuartLittlefair/astropy,joergdietrich/astropy,funbaker/astropy,DougBurke/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,larrybradley/astropy,astropy/astropy,tbabej/astropy,lpsinger/astropy,astropy/astropy,dhomeier/astropy