This methods ensures n_e(ψ) is everywhere positive and C²-continuous.
The density boundary is now dependent on the width w of the mollifier,
which also controls the smoothness of the curve, specifically:
ψ_bnd = ψ_last + w
where ψ_last = 1.01 currently is an extra point added to improve the
convergence of the mollified density to the original data as w→0.
3a2dd3697f
