Export roi meshes for neuprint

[stuarteberg]

Technically ROI meshes aren't part of the neuprint export per se, since they reside in DVID (typically in an instance named roisSmoothedDecimated.

But neuprint relies on those meshes being present, so this repo seems like a natural place to implement the mesh generation. The meshes don't need to be updated every time a snapshot is generated, but there should be an option to update them in the config somewhere.

And export them to disk for the "flat files" export.