Adding partitioned rates figure.

We used robust counting, might as well have a figure on it.
It shows synonymous, non-synonymous and nucleotide substitution rates
under different PB1-PB2-HA constellations.
M1 and NS1 were excluded due to having overlapping ORFs which might
mess with syn/nsyn rates.
Horizontal bars indicate non-overlapping HPDs. I'm not convinced the
significantly different ones are meaningful.
Minor text changes, beginning section on SPR and dTMRCA results.