Update TwoPowerSphericalPotential.py

[gusbeane]

Adds R2 deriv function for TwoPowerSphericalPotential

I tested against the Rforce function using some numerical derivatives, test suite should work. sample code below:

from galpy.potential import TwoPowerSphericalPotential
pot = TwoPowerSphericalPotential() 
assert (pot.R2deriv(1, 0.5) + (pot.Rforce(1+1E-8, 0.5) - pot.Rforce(1-1E-8, 0.5)) / 2E-8 ) < 1E-8
assert (pot.R2deriv(3, 0) + (pot.Rforce(3+1E-8, 0) - pot.Rforce(3-1E-8, 0)) / 2E-8 ) < 1E-8 

[jobovy]

Amazing, thanks! The travis runs take a while these days (because travis is winding down their free support for open source...), but I think the test suite will fail because TwoPowerSphericalPotential's lack of second derivatives was previously used to test some errors that should happen for potentials without second derivatives. I'll push some updates to those tests soon to fix that issue and then hopefully we can merge this quickly.

[jobovy]

My travis build is not starting, but it looks like it ran on your account and that all tests pass with the same coverage, so I'm just going to go ahead and merge this. Thanks again!

[gusbeane]

Great, thanks!