Add _Rzderiv to TwoPowerSphericalPotential

[gusbeane]

Sorry for making two PRs, didnt realize I also needed Rzderiv in my previous one 😅

here's a code tidbit for testing it numerically

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

[codecov]

Codecov Report

Merging #440 (d6db971) into master (c8029ee) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #440   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files         178      178           
  Lines       25443    25451    +8     
=======================================
+ Hits        25398    25406    +8     
  Misses         45       45           

Impacted Files	Coverage Δ
galpy/potential/TwoPowerSphericalPotential.py	100.00% <100.00%> (ø)

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[jobovy]

Thanks, that's great, all looks good