Some questions in acceleration.cpp and orbitProp.cpp

[yanspike]

There are some questions in acceleration.cpp and orbitProp.cpp

1. In accelSPH Function, when taking the derivative with respect to theta, it seems that in the expression for dV_theta_n, the factor of cos(theta) may have been omitted.
2. In OrbitIntegrator::computeAcceleration egm_field, the method used in the code to compute the derivative via finite differencing may lack sufficient accuracy. The precision of the derivative depends heavily on the chosen perturbation step size (posOffset), which can introduce numerical errors or instability.