Replace indices in CHILD functor with decay selector

[roelaaij]

As part of the LoKi functors lesson, the CHILD functor is taught with indices to select the child. As this depends on the ordering of the decay descriptor, it is discouraged. Perhaps the indices could be replaced with with decay selectors?

This brings its own complications, as (some minimal version of the) the grammar would have to be explained. At the same time, this is already part of the DTT lesson. Perhaps the lessons can be reordered to have the DTT lesson first up to the HybridTupleTool part, then the LoKi functors lesson, and then return to the DTT with the newly gained insights.

This might have the advantage that some "magic" particular to GaudiPython, e.g. explicit creation of the LoKi::DistanceCalculator can be avoided.