Add units to the different Vec3 "types" (Meters and Index, so far)

src/render.rs

@@ -1,5 +1,5 @@
 use ansi_term::Color::RGB;
-use crate::geom::{Vec2, Vec3};
+use crate::geom::{Meters, Vec2, Vec3};
 use crate::{flt::{Flt, Real}, Material as _};
 use crate::mat;
 use crate::sim::{Cell, GenericSim};
@@ -92,7 +92,7 @@ impl<'a> RenderSteps<'a> {
         let y_prop = y_px as Flt / self.im.height() as Flt;
         let y_m = y_prop * (self.sim.height() as Flt * self.sim.feature_size());
         let z_m = self.z as Flt * self.sim.feature_size();
-        self.sim.sample(Vec3::new(x_m, y_m, z_m))
+        self.sim.sample(Meters(Vec3::new(x_m, y_m, z_m)))
     }
 
     //////////////   Ex/Ey/Bz configuration ////////////