AP_Math: make vectors and matrix constructors constexpr

That allows some object to be constructed at compile time.

libraries/AP_Math/matrix3.h

@@ -50,16 +50,21 @@ public:
 
     // trivial ctor
     // note that the Vector3 ctor will zero the vector elements
-    Matrix3<T>() {
-    }
+    constexpr Matrix3<T>() {}
 
     // setting ctor
-    Matrix3<T>(const Vector3<T> &a0, const Vector3<T> &b0, const Vector3<T> &c0) : a(a0), b(b0), c(c0) {
-    }
+    constexpr Matrix3<T>(const Vector3<T> &a0, const Vector3<T> &b0, const Vector3<T> &c0)
+        : a(a0)
+        , b(b0)
+        , c(c0) {}
 
     // setting ctor
-    Matrix3<T>(const T ax, const T ay, const T az, const T bx, const T by, const T bz, const T cx, const T cy, const T cz) : a(ax,ay,az), b(bx,by,bz), c(cx,cy,cz) {
-    }
+    constexpr Matrix3<T>(const T ax, const T ay, const T az,
+                         const T bx, const T by, const T bz,
+                         const T cx, const T cy, const T cz)
+        : a(ax,ay,az)
+        , b(bx,by,bz)
+        , c(cx,cy,cz) {}
 
     // function call operator
     void operator        () (const Vector3<T> &a0, const Vector3<T> &b0, const Vector3<T> &c0)

libraries/AP_Math/vector2.h

@@ -38,13 +38,14 @@ struct Vector2
     T x, y;
 
     // trivial ctor
-    Vector2<T>() {
-        x = y = 0;
-    }
+    constexpr Vector2<T>()
+        : x(0)
+        , y(0) {}
 
     // setting ctor
-    Vector2<T>(const T x0, const T y0) : x(x0), y(y0) {
-    }
+    constexpr Vector2<T>(const T x0, const T y0)
+        : x(x0)
+        , y(y0) {}
 
     // function call operator
     void operator ()(const T x0, const T y0)

libraries/AP_Math/vector3.h

@@ -68,13 +68,16 @@ public:
     T        x, y, z;
 
     // trivial ctor
-    Vector3<T>() {
-        x = y = z = 0;
-    }
+    constexpr Vector3<T>()
+        : x(0)
+        , y(0)
+        , z(0) {}
 
     // setting ctor
-    Vector3<T>(const T x0, const T y0, const T z0) : x(x0), y(y0), z(z0) {
-    }
+    constexpr Vector3<T>(const T x0, const T y0, const T z0)
+        : x(x0)
+        , y(y0)
+        , z(z0) {}
 
     // function call operator
     void operator ()(const T x0, const T y0, const T z0)