`set_agepro_model` helper method

[efletcherPIFSC]

Creating a helper method set_agepro_model, which (by default) instantiates a new agepro_inp_model class, will help resolve:

• Allowing users new to ageproR, to create or set up new agepro_model classes without the need to learn R6class instantiation.
• Straightforward example documentation.