Outsource some of the FlyWire functions to a separate package

[schlegelp]

Maybe navis-graphene? That way we can more easily reuse code for FANC.

Alternatively, we could widen the scope of fafbseg and rename it to e.g. navis-emdatasets with modules for flywire, fafb, fanc and hemibrain.

navis-flyem, navis-fly, navis-flydata