add metadata attr to Cosmo
add metadata attr to Cosmo

Signed-off-by: Nathaniel Starkman (@nstarman) <ac14c573519de521a9fc5a84607d51c0c956190b@protonmail.com>

astropy/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,saimn/astropy,lpsinger/astropy,mhvk/astropy,pllim/astropy,mhvk/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,pllim/astropy,aleksandr-bakanov/astropy,mhvk/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,saimn/astropy,mhvk/astropy,saimn/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,saimn/astropy,lpsinger/astropy,larrybradley/astropy,saimn/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,pllim/astropy,aleksandr-bakanov/astropy,astropy/astropy,aleksandr-bakanov/astropy