function add(a, b, dst) {
  dst = dst || new VecType(3);
  dst[0] = a[0] + b[0];
  dst[1] = a[1] + b[1];
  dst[2] = a[2] + b[2];
  return dst;
}
Adds two vectors; assumes a and b have the same dimension.
@param {module:twgl/v3.Vec3} a Operand vector.
@param {module:twgl/v3.Vec3} b Operand vector.
@param {module:twgl/v3.Vec3} [dst] vector to hold result. If not new one is created.
@return {module:twgl/v3.Vec3} A vector tha tis the sum of a and b.
@memberOf module:twgl/v3