title: Rotate a quaternion / Change the axis it rotates around
body: <p>Conceptually a quaternion can store the rotation around an axis by some degree.
  Now, what is the numerically most robust and least calculation intensive way to rotate the axis?</p>
  <p>For example when i have a quaternion, which rotates 90 degrees around the y-axis and i want those 90 degrees to be applied to some other arbitrary axis, described by a normalized vector.</p>
  <p>EDIT: Since this also came up i added an answer on how to correctly change the axis a quaternion rotates around with another quaternion.</p>