now all ics modules creates instances of ParticleSystem class.
attempts toward a more general get-/set-attribute for ParticleSystem class.
fix bug in ParticleSystem.__getattr__ introduced in the previous commit.
attempt to add a time-symmetric recursive method in SIA.
merge several files containing ICs for few-body systems into a single fewbody.py module.
Added ICs for the solar system. Also, minor bug fixes.