Change ._frame attribute to .local
Change ._frame attribute to .local

There are situations when you need to directly access the DataFrame
underlying a DataFrameWrapper, especially when trying to shadow that
column with a computed column. Instead of forcing people to use
the semi-private ._frame now it's .local.

bricegnichols/urbansim,synthicity/urbansim,ual/urbansim,synthicity/urbansim,AZMAG/urbansim,SANDAG/urbansim,UDST/urbansim,apdjustino/urbansim,AZMAG/urbansim,ual/urbansim,SANDAG/urbansim,bricegnichols/urbansim,VladimirTyrin/urbansim,VladimirTyrin/urbansim,apdjustino/urbansim,synthicity/urbansim,ual/urbansim,bricegnichols/urbansim,synthicity/urbansim,UDST/urbansim,AZMAG/urbansim,waddell/urbansim,bricegnichols/urbansim,waddell/urbansim,waddell/urbansim,UDST/urbansim,apdjustino/urbansim,VladimirTyrin/urbansim,UDST/urbansim,apdjustino/urbansim,VladimirTyrin/urbansim,SANDAG/urbansim,waddell/urbansim,SANDAG/urbansim,AZMAG/urbansim,ual/urbansim