differentiable Sersic

[ismael-mendoza]

@rmjarvis mentioned this issue might be useful:

• https://github.com/GalSim-developers/GalSim/issues/566

And this math stackexchange question:

• https://math.stackexchange.com/questions/3808975/is-there-an-analytic-formula-or-good-fitting-function-for-the-hankel-transform-o