Generalise HH truth decay algorithm

Updated HH truth decay algorithm to be configured depending on targeted decay modes for the HH pair (a vector is supported in case multiple ones are to be used). Sets the ID of the children particle to consider depending on this decay mode.

As of now, this is still simple and generic, so all decay modes can be covered by a single algorithm. If some extra refinements are found to be needed, one could consider this as a base class and use inheritance to refine the getFinalChildren function depending on the decay mode.

Solves #82 (closed)

Checked to provide reasonable outputs for new decay modes introduced ~bbtautau and ~bbyy. Note that no particular ordering of the Higgs by decay mode is implemented at this stage, to keep this generic enough towards potential future extension for additional HH decay modes. If needed, this can be treated downstream or potentially with new configurable properties.