Define a Torus region.

I mean, actual core memories are usually toroids, so...

src/geom/vec.rs

@@ -179,6 +179,16 @@ impl Vec3 {
             z: self.z / other.z,
         }
     }
+
+    pub fn with_mag(&self, new_mag: Flt) -> Vec3 {
+        if new_mag == 0.0 {
+            // avoid div-by-zero if self.mag() == 0 and new_mag == 0
+            Self::zero()
+        } else {
+            let scale = Real::from_inner(new_mag) / self.mag();
+            self.clone() * scale.into_inner()
+        }
+    }
 }
 
 impl Into<(Flt, Flt, Flt)> for Vec3 {