Gas ring model: sun position

[carmeloevoli]

The gas ring model cannot be used with an observer position different than [8.5,0,0]. An exception should be raised if not.

[adundovi]

This can be solved by introducing checks in the integrators' constructors which accepts the gas ring model. The ring model would provide its requirements (like the observer position) and the integrator would use them to check if everything is O.K. with other settings. But I would leave this change after the release of 1.0 as it is more like a new feature.