code: def _mass(self,R,z=0.,t=0.):
          
          if z is None: r= R
          else: r= numpy.sqrt(R**2.+z**2.)
          return numpy.log(1+r/self.a)-r/self.a/(1.+r/self.a)
docs: NAME:
             _mass
          PURPOSE:
             calculate the mass out to a given radius
          INPUT:
             R - radius at which to return the enclosed mass
             z - (don't specify this) vertical height
          OUTPUT:
             mass in natural units
          HISTORY:
             2014-01-29 - Written - Bovy (IAS)