ENH: Attach recombinants at most recent common ancestor

[corneliusroemer]

This way we don't have the artificial non-specific clade "recombinant"

For example, XBB is derived from 21L and 22D - 22D derives from 21L, so XBB should be 21L, and attach at 21L, or a more recent common ancestor.

Importantly, the branch length to recombinants needs to be long, so that the ancestral reconstruction is not misled.

It would be nice if it was possible to visually indicate recombinants in Auspice, e.g. by having dashed line instead of full line.