Optional inputs and ouputs enabling

[RaluAlexa]

At the moment, declaring x inputs or outputs in the config means all x need to be returned in the define functions and all outputs are expected after the model run.
Enabling optional outputs will help with cases when the outputs (with different schemas) are dependent on model parameters, and a single one (or subset of all) need to be returned (e.g. scenario runs). Currently this needs to be handled from inside the model code, with conditional outputting based on parameters and empty dataframes being returned for the outputs not produced by that run.

[martin-traverse]

Duplicate of #137