table: move data attribute back into Column object
table: move data attribute back into Column object

Previously there was concern about double-storing column
data if it was held as an attribute of Column.  However,
without data it isn't possible to fully initialize a table
with a list of column objects.  In this implementation
the private _data attribute is set to None as soon as a
parent table reference is set, thus releasing the reference.
After the reference is set col.data returns the view
of the appropriate parent table column.

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