Use common name to match species during tree import.

Fixes #1847
I don't quite understand this assertion, probably because I am not intimately familiar with the internals of the importer. We have coordinates for all three rows, so I am naively assuming that the ``TreeImportEvent`` would have a non-None plot. Is that just how things work that, if we can't match the species, then we don't get any ``Plot`` data on the ``TreeImportEvent``?