Backfill some tests for the LinearMagnet which show that it's behaving POORLY

src/mat/linear.rs

@@ -87,6 +87,11 @@ mod test {
 
         mag.step_b(&CellState::default(), Vec3::uniform(1.0));
         assert_float_eq!(mag.m().x(), 1591231.12, abs <= 1.0);
+
+        mag.step_b(&CellState::default(), Vec3::uniform(-1.0));
+        assert_float_eq!(mag.m().x(), 795615.56, abs <= 1.0);
+        mag.step_b(&CellState::default(), Vec3::uniform(-1.0));
+        assert_float_eq!(mag.m().x(), 0.0, abs <= 1.0);
     }
 
     #[test]
@@ -99,4 +104,24 @@ mod test {
         mag.step_b(&CellState::default(), Vec3::uniform(-3.0));
         assert_float_eq!(mag.m().x(), -795774.72, abs <= 1.0);
     }
+
+    #[test]
+    fn linear_magnet_accuracy() {
+        let mut mag = LinearMagnet::<f32>::new(5000.0);
+
+        let mut b = Vec3::zero();
+        while b.x() < 1.0 {
+            let delta_b = Vec3::uniform(0.00002);
+            mag.step_b(&CellState::default(), delta_b);
+            b += delta_b;
+        }
+        while b.x() > 0.0 {
+            let delta_b = Vec3::uniform(-0.00001);
+            mag.step_b(&CellState::default(), delta_b);
+            b += delta_b;
+        }
+        // TODO: This error is WAY too big! 
+        // Need to make sure that M+H == mu0*B always
+        assert_float_eq!(mag.m().x(), b.x() * f32::mu0_inv(), abs <= 900.0);
+    }
 }