Ensure codified entries always remain

[rhagenson]

As identified during review by openjournals/joss-reviews#2124 (comment), relped is currently dropping some codified entries such as half-siblings. I am uninterested in precisely "why" these relationships are being dropped as I suspect it is a case of pruning and the order that pruning happens to occur in for a particular iteration.*

What is important here is to match user expectation where if they have entered a code, then that connection should always exist in the final pedigree network.

*: If two paths have the same weight because they were both drawn from the same code relped during pruning selects the "highest weight" path then drops the others even if equally weighted.