Math: removed constexpr from non-trivial Quaternion methods.

They will probably involve SIMD operations, which cannot be implemented
as constexpr.
Doc++
Math: test also properties of normalized Quaternion.
Math: "checked" function for inverting normalized Quaternion.
Math: using absolute #includes in Quaternion.h.
Math: creating Quaternion from axis/angle.
Math: converting Quaternion to rotation matrix.
Math: function for negating Quaternion, improved tests.
Doc++

 * Added math equations to Quaternion, Vector and Matrix method
   documentation.
 * Removed confusing Quat*=Quat operator overload, as it isn't exactly
   clear from which side the non-commutative multiplication is done:

    Quaternion a;
    a *= b;  // eh?
    a = a*b; // okay!

   For similar reason this operator wasn't present in RectangularMatrix
   either.
 * Unified documentation of expected vector/quaternion normalization
   state. Now it is not "assumed" but "expected", because failing to do
   so results in assertion failure.