code: def R(self,*args,**kwargs):
          
          thiso= self(*args,**kwargs)
          onet= (len(thiso.shape) == 1)
          if onet: return thiso[0]
          else: return thiso[0,:]
docs: NAME:
             R
          PURPOSE:
             return cylindrical radius at time t
          INPUT:
             t - (optional) time at which to get the radius
             ro= (Object-wide default) physical scale for distances to use to convert
             use_physical= use to override Object-wide default for using a physical scale for output
          OUTPUT:
             R(t)
          HISTORY:
             2010-09-21 - Written - Bovy (NYU)