Return the completeness as a function of magnitude.

        ADW: Eventually want a completeness mask to set overall efficiency.

def completeness(self, mags, method='step'):
        """
        Return the completeness as a function of magnitude.

        ADW: Eventually want a completeness mask to set overall efficiency.
        """
        if method == 'step':
            func = lambda x: (x < self.mask_roi_unique[:,np.newaxis]).astype(float)
        elif method == 'erf':
            # Trust the ERD???
            # 95% completeness: 
            def func(x):
                # Efficiency at bright end (assumed to be 100%)
                e = 1.0
                # SDSS EDR says full width is ~0.5 mag
                width = 0.2 
                # This should be the halfway point in the curve
                maglim = self.mask_roi_unique[:,np.newaxis]
                return (e/2.0)*(1/np.sqrt(2*width))*(np.sqrt(2*width)-scipy.special.erf((x-maglim)))
        else:
            raise Exception('...')

        return func(mags)