Code does not work with modern poliastro versions

[astrojuanlu]

Hi! I'm the creator of poliastro (see also gh-16). It has been brought to my attention that spacetech-ssa doesn't work with modern poliastro versions.

For example, the force models are called like this:

spacetech-ssa/orbit_prediction/orbit_prediction/physics_model.py

Lines 96 to 100 in 26986e5

	prop_orbit = orbit.propagate(s*u.s,
	method=cowell,
	ad=J2_perturbation,
	J2=Earth.J2.value,
	R=Earth.R.to(u.km).value)

However, that syntax was changed in poliastro 0.15, and then again in poliastro 0.17. See this example for more info:

https://docs.poliastro.space/en/0.17.x/examples/Natural%20and%20artificial%20perturbations.html

Would you accept pull requests to fix that part of the code for newer poliastro versions, which are also faster?

cc @Kube008