adding docs on variables.py
adding docs on variables.py

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