Support over-riding zero-based status of PstFrom obj in add_pars (or obs) calls

Bit of de ja vu here. Maybe this has been raised before. 

Some custom projects can end up with mixtures of zero-based and one-based indexing in simulation files. For e.g. if using different simulators with different requirements. Might be handy to add an override option to add_paramters and add_observation calls to support a bit of fine tuning. 