IsothermalDiskPotential amplitude error

[morganb-phys]

The code below shows that when you call IsothermalDiskPotential, the second derivative does not give the amplitude (density) entered when the potential is initialized. However, pot._evaluate() does give the correct amplitude.

from galpy.potential import IsothermalDiskPotential,turn_physical_off
import numpy as np

z= np.linspace(-0.1,0.1,10001)
pot= IsothermalDiskPotential(amp=0.1, sigma=20.5/220.)
turn_physical_off(pot)

p1= 1./(np.pi*4)*np.gradient(np.gradient(pot(z),z),z)
p2= 1./(np.pi*4)*np.gradient(np.gradient(pot._evaluate(z),z),z)

print(p1[5000]) #Result is: 0.01
print(p2[5000]) #Result is: 0.1