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

[briochh]

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.

[jtwhite79]

We probably need to even support a column-level option in the case of multiple index columns...

[briochh]

Happy for anyone else to take this on...