added multiplication for Kartesians

some part of it had to be an new trait

src/coordinate_systems/kartesian/external_traits.rs

@@ -149,3 +149,13 @@ impl<T: Integer + Display + Unsigned + Copy> Display for Velocity<T> {
         Ok(())
     }
 }
+
+impl<T: Integer + Mul> Mul for Kartesian<T> {
+    type Output = Kartesian<T>;
+    fn mul(self, rhs: Self) -> Self::Output {
+        Kartesian {
+            x: self.x * rhs.x,
+            y: self.y * rhs.y,
+        }
+    }
+}

src/coordinate_systems/kartesian/traits.rs

@@ -67,3 +67,29 @@ impl ToCoords for Kartesian<usize> {
         self
     }
 }
+
+pub trait IntMul<T: Integer> {
+    type Output;
+    fn mul(self, rhs: T) -> Self::Output;
+}
+
+macro_rules! impl_mul {
+    ($subtype:ident) => {
+        impl IntMul<$subtype> for Kartesian<$subtype> {
+            type Output = Kartesian<$subtype>;
+            fn mul(self, rhs: $subtype) -> Self::Output {
+                Kartesian {
+                    x: self.x * rhs,
+                    y: self.y * rhs,
+                }
+            }
+        }
+    };
+}
+
+impl_mul!(u8);
+impl_mul!(u16);
+impl_mul!(u32);
+impl_mul!(u64);
+impl_mul!(u128);
+impl_mul!(usize);