examples: improve the pendulum simulation, with several modes and diagrams (#13446)

examples/pendulum-simulation/modules/sim/vec.v

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+module sim
+
+import math
+
+// Vector3D is a 3D vector
+pub struct Vector3D {
+	x f64
+	y f64
+	z f64
+}
+
+// vector creates a Vector3D passing x,y,z as parameteres
+pub fn vector(data Vector3D) Vector3D {
+	return Vector3D{
+		...data
+	}
+}
+
+// addition
+pub fn (v Vector3D) + (v2 Vector3D) Vector3D {
+	return Vector3D{
+		x: v.x + v2.x
+		y: v.y + v2.y
+		z: v.z + v2.z
+	}
+}
+
+// dot product
+pub fn (v Vector3D) * (v2 Vector3D) f64 {
+	return (v.x * v2.x) + (v.y * v2.y) + (v.z * v2.z)
+}
+
+// scale gets a scaled vector
+pub fn (v Vector3D) scale(scalar f64) Vector3D {
+	return Vector3D{
+		x: v.x * scalar
+		y: v.y * scalar
+		z: v.z * scalar
+	}
+}
+
+// norm_squared returns the square of the norm of the vector
+pub fn (v Vector3D) norm_squared() f64 {
+	return v * v
+}
+
+// norm returns the norm of the vector
+pub fn (v Vector3D) norm() f64 {
+	return math.sqrt(v.norm_squared())
+}