Feature Request: Saving unfinished flatbush index

[lsarrazi]

Im using flatbush on 100M items, my index take 1-2h to build (mostly my database connection speed)

I would want to save my flatbush data unfinished for later completion, unfortunately currently it's not possible to load it back with Flatbush.from nor the constructor with a buffer. The problem since to be _pos which is not saved anywhere, and somewhat initialized from numItems which is not correct since the flatbush isn't completed

Would it be possible ?