Remove PML

There are still a few references to it, but it doesn't really exist in
the simulation proper.

The existing implementation is confusing and ineffective (broken?).
Upcoming patches will work to replace it with a more workable
implementation.

Also moves the Material::is_vacuum method to be on a helper trait.
That's just a point of hygeine. Meant to split it out to a separate
patch but messed up (-:

src/render.rs

@@ -1,5 +1,5 @@
 use crate::geom::{Index, Meters, Vec2, Vec3, Vec3u};
-use crate::{flt::{Flt, Real}, Material as _};
+use crate::{flt::{Flt, Real}, Material as _, MaterialExt as _};
 use crate::mat;
 use crate::sim::{Cell, GenericSim, StaticSim};
 use crate::meas::AbstractMeasurement;