This sets the unit that the attribute will be shown with. It
uses the same mechanism that's used to remember what units
you had selected.

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