integrate with planned hubUtils model_outputs class

[elray1]

Once it is implemented in hubUtils, we should request that the input to simple_ensemble is an object of class model_output or convert it to one if it is not, and convert the result to this class before returning.

Related hubUtils issue: https://github.com/Infectious-Disease-Modeling-Hubs/hubUtils/issues/66

[elray1]

This class now exists, documented at: https://infectious-disease-modeling-hubs.github.io/hubUtils/reference/as_model_out_tbl.html

We should:

• convert the model_outputs argument to simple_ensemble to a model_out_tbl at the beginning of the function if it is not one already.
• Note that this will automatically perform some of the validation steps that are currently done at the beginning of simple_ensemble, so we will then be able to delete any redundant checks from simple_ensemble.
• Also convert the ensemble_model_outputs to a model_out_tbl before returning.