Conjugate and inverse

Games mostly use normalized quaternions, which comes in handy when inverting quaternions. The inverse of a normalized quaternion is its conjugate. The conjugate

of a quaternion flips its axis of rotation:

1. Implement the conjugate function in quat.cpp and remember to declare the function in quat.h:

   quat conjugate(const quat& q) {
       return quat(
           -q.x,
           -q.y,
           -q.z,
            q.w
       );
   }

2. The proper inverse of a quaternion is the conjugate divided by the squared length of the quaternion. Implement the quaternion inverse function in quat.cpp. Add the function declaration to quat.h:

   quat inverse(const quat& q) {
      float lenSq = q.x*q.x + q.y*q.y + q.z*q.z + q.w*q.w;
      if (lenSq < QUAT_EPSILON) { 
    ...