combineasy and the DiseasyEnsemnle class

[RasmusSkytte]

Intent

This PR adds helpers to creating and managing model ensembles

Approach

This Pr Adds the combineasy() helper function which injects parameters and functional modules into model templates to create the model ensemble.

A model ensemble is here a list of model instances which we give the class DiseasyEnsemble.

By giving the class to list, we can now create the standard generics print, summary and plot which allows us to inspect the ensemble and create plots from it.

Known issues

A predict generic should probably be added and used internally in plot.

Missing fig.alt text in creating-ensembles.Rmd

Gracefully handle missing suggests in creating-ensembles.Rmd

plot() example is missing arguments

Waiting on

• Add vignette and plotting methods for DiseasyModelRegression #164

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR