code: def surfacemassDerivative(self,R,log=False):
          
          if log:
              return -1./self._params[0]
          else:
              return -sc.exp(-R/self._params[0])/self._params[0]
docs: NAME:
             surfacemassDerivative
          PURPOSE:
             return the derivative wrt R of the surface density profile at this R
          INPUT:
             R - Galactocentric radius (/ro)
             log - if True, return the derivative of the log (default: False)
          OUTPUT:
             Sigma'(R) or (log Sigma(r) )'
          HISTORY:
             2010-03-26 - Written - Bovy (NYU)