Minor changes for usability of LRGV

These two commits:

1. Add an example python script for interacting with LRGV, which might form the basis for other applications

2. Modifies how help on objectives and constraints is given.
The available objectives and constraints are defined in vectors obj_avail and constr_avail in lrgv_Simulation.cpp, so they can be listed in the help, and so the settings specified in the control file can be validated.
This only slows down input of the control file and display of help, so it should not have any impact on speed of the main model run.
The only complication is that new objectives and constraints need to be added to those vectors to be recognised, but this seems simpler than having to keep documentation of available objectives and constraints up to date (even the comments in the code can easily get out of date).