Synonyms in FAFB have unexpected values

[floesche]

For example, 720575940605709024 (R7) has the synonym L1. There are many other similarly weird synonyms. Check against mapping file and explain the issue in more detail.

This was initially raised in #16 for the flywire-fafb:v783b dataset

[floesche]

After checking in with Stuart, he will delete the synonyms properties from the FAFB dataset. At some point, there will be a MCNSType property, based on https://flyem.mrc-lmb.cam.ac.uk/flyconnectome/mappings/mcns_fw_mapping.json, that will allow backlinks from the FAFB to the MCNS dataset. Until then, we don't have those links.