Separate the logic that combines matching allele pairs into combined standards in `HLAAlgorithm.combine_standards`

[rhliang]

While working on the BBLab HLA tool, we see that this will make things more flexible and able to work in the "streaming" fashion that the BBLab tool currently does. We'd already refactored combine_standards so that the labour-intensive part is done by a generator in preparation for it working with the BBLab tool; separating out this logic will allow the BBLab code to combine the generated results in a manner that makes sense for it.