Expand/change Ensemble to include AbsEndcap

[GilesStrong]

Current state

AbsEndcap acts as a wrapper to apply fixed functions to the outputs of models that were trained on proxy objectives, e.g. to compute the invariant mass from a model that outputs the 3-momenta of two particles (see Multi_Target_Regression example).

AbsEndcap currently does this via a .forward method and a .predict method, so in theory, this should also work for Ensembles of models. Wrapping an Ensemble, though, then removes many of the benefits of that the Ensemble class offers.

Proposal

The Ensemble class is extended to include the ability to add an AbsEndcap, through which the outputs of all internal models will be passed.