Add fn about the math of aggregation

[rfl-urbaniak]

          Made significant revisions to section 6 on evidence aggregation (simple case). The section seems to be fine, more or less, perhaps it needs to be shortened. But I have to requests:

1. I think we need a general formula that shows how two items of independent match evidence (and their higher-order uncertainties) are aggregated. In precise probabilism, you multiple the LRs, in higher order probabilism what do you? We have a Figure that shows a join higher order distribution, but what is the formula that gives that distribution. Just ,multiply LRs whose terms are densities and not precise probabilities? Is that it?

2. I want to know whether, for evidence assessment, higher order probabilism and precise probabilism will come into conflict. is the case that the the combined LR of two match evidence (LR1XLR1) in precise probabilism is different from the mean of the joint joint density of the two matches?

I still need to work on the final section 7: evidence aggregation (complex case).

Originally posted by @marcellodibello in https://github.com/rfl-urbaniak/MRbook/issues/10#issuecomment-1909197053

[rfl-urbaniak]

talk about difference in LR ratios