Support for BWAmem GRCh38 alt workflow

[genomematt]

Modifications to deal with ALT hits flagged with 0x800 (supplementary) and a variable number of mappings for the same read id.
Xenomapper needs to collect all these as a group, use the maximum score to compare to mouse, and allocate them to the correct read type as a block.
Tests should include cases of unique on alt and multimapping on alt