Added definitions and Figures of Merit section

Merged Jose Marquez Prieto requested to merge eln_baseclasses_hzb_stability_fom into eln_baseclasses_hzb

I have included some definitions and drafted a figure of merit section. Ideally, we would have an algorithm that extracts these parameters automatically from the time-dependent metrics. This is not as trivial as it might look in the beginning and would be great to discuss it. It would also bring a way to report stability figures of merit in a common standardized manner across the labs we support. @g-michaelgoette feel free to copy, merge or take whatever you consider from here. If you do not merge, please close the MR and delete the branch afterward.

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