Fix NAMESPACE permissions for agepro_model

[efletcherPIFSC]

Limit agepro_model values and functions are added to the NAMESPACE.

[efletcherPIFSC]

Current R6class implementation of agepro model structure will make this infeasible. A workaround to this is to introduce classless helper functions that can return agepro_inp_model and agepro_json_model (#101)