Inkscape now exits before it finishes writing its files, somehow.
So, we need to wait for the files to be created. The Blender
add-on also now correctly configures the lineart to use the
secondary camera, which is rotated to avoid errors when rendering
orthographically.

Also update the add-on to allow for configuration of
potrace's alpha settings, which controls how strongly
it favors straight lines. This makes things like buildings
look a lot nicer.

Also, modify the Blender add-on so that mass can be
computed via density even if a volume is not being included

Also update the add-on a bit to fix some errors

Also update the scripts to support tagging entities
with masses and volumes.

Also updates the add-on some more. It can now
assign materials based on face maps, which the
Building Tools add-on creates automatically