Producing a mesh for a single material (isosurfacing)

[jalevine]

If provided with a single material, it would be handy to specify and isovalue and have cleaver default to just running something like isosurface stuffing.

Essentially, this would create a second "virtual" material of constant value (whatever the specified isovalue). The net result is that cleaver would find where the two materials (the input and the virtual) were equal --- producing an isosurface.

I think a feature like this was implemented at some point, but if not it's a small modification that allows cleaver to be both an isosurfacer that also builds a tet mesh for the isosurface too.