Revisit B3.13 rooting & clock rates

* There are now enough data for a temporal rooting I think, the clock signal seems strong. * We should revisit the strains (if any) being excluded by our current clock filter * [Pekar et al (Virological)](https://virological.org/t/timing-and-molecular-characterisation-of-the-transmission-to-cattle-of-h5n1-influenza-a-virus-genotype-d1-1-clade-2-3-4-4b/991) report a depressed clock rate for B3.13 cf. background 2.3.4.4b. We should try this rate out, or even attempt to estimate it ourselves.