fix setters of p0c and delta to update _rvv and zeta

[roman-martin]

Fix for issue #55

However, I am not 100% pleased with the current status:

• For one, I am not sure if there are other parameters to be updated as well (_rpp?).
• The next problem is that _update_particles_from_absolute() is now updating zeta, but relatively, not from absolute like the function name suggests... we might want to create a differently named function for that?
• Lastly the delta.setter now has to check whether the attribute _rvv already exists (as it is also used during initialization) in order to update zeta. Again we might want to change the program flow a bit more here.