(more) deterministic weighted relations ordering

In case a 'from' object is related to multiple 'to' objects, the ordering of the 'to' objects is based on the (value of the) weight. When multiple 'to' objects have the same weights, this MR insures that the order for those 'to' objects is deterministic by ordering them according to their keys (provided they are in the same container -- if not, use the lexicographic order of the containers)

In addition, tweak the relations 'entries' classes such that there is no need for mutable 'to' and 'from' members -- both the referred-to 'to' and 'from' entries should reside in the event store, and thus, by construction, there should be no need to have 'non-const' access to them.

requires Lbcom!692 (merged), Boole!515 (merged), Rec!3572 (merged), DaVinci!962 (merged) related to Moore!2632 (merged) LHCbIntegrationTests!48 (merged) DaVinci!953 (merged) Moore!2636 (merged)