code: def surfacemass(self,R,log=False):
          
          if log:
              return -R/self._params[0]
          else:
              return sc.exp(-R/self._params[0])
docs: NAME:
             surfacemass
          PURPOSE:
             return the surface density profile at this R
          INPUT:
             R - Galactocentric radius (/ro)
             log - if True, return the log (default: False)
          OUTPUT:
             Sigma(R)
          HISTORY:
             2010-03-26 - Written - Bovy (NYU)