Spike only sequences from VOC alpha aren't assigned the 20I (Alpha) but 20B

[rneher]

The presence of recombinant lineage XA with Alpha spike in a non-alpha background attracts Spike only Alpha sequences.

Unsure how to handle this, but as a stop-gap solution we could remove recombinant lineages from the reference tree.

[corneliusroemer]

Since I've implemented only the stop-gap solution, I'll reopen and label with hard problem.