Draft: Algorithm to enable the functionality of saving isolation MVA output in a RelationTable for SL

Implementation of the algorithm that creates RelationTable to persist and be useful for SL isolation MVA.

Input: container of B^{*+}, container of signal candidates and F.MVA functor

Output: Relation table <signal, extra track, MVA output>.

Application of the algorithm in Moore!4109

Addition of ELEMENT_AT_INDEX functor with proper tests

NEEDS LHCb!4879 (merged), LHCb!4880 (merged), LHCb!4872

• Test

cc: @amathad, @anmorris, @chinghua, @mrudolph

FYI: @graven, @sstahl, @mveghel, @decianm, @mfontana, @nskidmor