Adds a way to create an ensemble out of existing models

[Craigacp]

Description

This PR adds a method to WeightedEnsembleModel called createEnsembleFromExistingModels which builds an ensemble out of the supplied model list, ensemble combiner and optionally model combination weights. It adds a TimestampTrainerProvenance which is used as the TrainerProvenance for any ensembles created this way. Finally it adds a couple of tests for VotingCombiner and AveragingCombiner along with a few tests of the new behaviour.

The ensemble building mechanism validates that there are at least two models, that there are equal numbers of models and combination weights and that the output domains for all the supplied models are the same. We could consider adding a check that the feature domains are the same, or have some overlap, but it's plausible that text models trained using data subsampling have different views of the feature space and so I've left that check for future work.

Motivation

It's currently hard in Tribuo to ensemble together models of different types (e.g. an ensemble of a linear model, a decision tree and a kernel SVM). This kind of arbitrary ensembling is occasionally very powerful, but difficult to express via Tribuo's trainer mechanism, so it's simpler to allow users to build custom ensembles from models they already have.

Paper reference

Kuncheva, L. I. (2014). Combining pattern classifiers: methods and algorithms.

[JackSullivan]

Mostly looks good, apart from a few questions/edits.

I'd like to entertain doing something better with the provenance than just taking the first model's feature domain. i.e., how involved would it be to create a DatasetUnionProvenance? We don't seem to need the actual union here, though.

[Craigacp]

Well the ensemble provenance contains each of the individual models' provenances, and those each contain their training dataset provenance. So the information is all there, but I needed to pick something to put in the top level model provenance and the first one seemed easiest.

[JackSullivan]

Looks good based on our discussion with the above changes.

[JackSullivan]

Looks good to me.