adds C/CL extensions to calculate 'snap' and 'crackle' derivatives of acceleration.
implements Hermite integrators of orders 2, 4, 6 and 8.
Hermite integrators of different orders are now implemented separately.
reorganizes methods in sia.py; also adds standard KDK-type methods.
Removes auxiliary attributes used in Hermite integrators. Instead, now a full copy of the particle system is the preferred way of keep the values of the attributes from the previous step.
Now hermite integration uses the Evaluate-Predictor-(Evaluate-Correct)n EPECn scheme.
Adds kdk-type methods and some improvements in sia.py. Also, adds a bug fix in hermite integrators.
slow/fast kicks now correctly evolves PN equations.
Replaces [:] -> [...] in numpy arrays. Also, removes the global variable in sia.py by storing it as a class attribute of ParticleSystem.