Improve the `Explicit` strategy

[ablaom]

This stategy (originally added for testing purposed only) is has not been publicised, but needs some improvements before doing so:

Problems:

• Currently the explicit list (or iterator) of models is specified as range and all models need to have the same type - which is extremely restrictive, as the main use case is for evaluating multiple models of different types.

• Currently the user has to specify some model=... , say the first in the list, which is clunky

Suggested resolution:

• Add a new keyword models=... whose specification flags the strategy automatically as Explicit, and whose value is the iterator of models to be compared. Internally models is copied to range. An error is thrown if models and range are both specified and not the same, or if model and models are both specified but model isa eltyype(models) is not true. This maintains backwards compatibility.

Implementation:

To get around the design issue requiring all models to have the same type, we can apply a thin wrapper to all models in the list and give a wrapped model the same setproperty!/getproperty interface as the original. The alternative (I'm guessing, from memory) is to remove the model type parameter M from MLJBase.Resampler{M} which could be painful, but worth checking as this would be less of a hack. Any loss of performance in dropping the type parameter is likely trivial in 99% of use cases.

[rikhuijzer]

The last part makes me conclude that this issue is a bit to tough for me. Let me know if I can help out (with documentation?).