Account for different segment numbers in Influenza B and Influenza A viruses

[peterk87]

See CFIA-NCFAD/nf-flu#65

PB1 is segment 1, PB2 is segment 2 in IBV.

PB2 is segment 1, PB1 is segment 2 in IAV.

source: https://viralzone.expasy.org/80