Check that quaternion is normalized when converting to rotation matrix

  The conversion formula used is valid only for unit quaternions.
  If a user passes a zero quaternion by mistake, the resulting matrix is
  identity. This is very confusing since multiplying by a Transform3f containing
  0 as a quaternion results in identity Transform3f.
Merge pull request #26 from florent-lamiraux/master

Check that quaternion is normalized when converting to rotation matrix