[math][GenVector] Add access operator for Rotation3D

[mdessole]

This Pull request:

Fixes https://root-forum.cern.ch/t/tvector3-rotate-to-arbitrary-rotation-using-xyzvector/63244

Alternative to #18462

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    17 files      17 suites   3d 15h 40m 4s ⏱️
 2 730 tests  2 730 ✅ 0 💤 0 ❌
45 035 runs  45 035 ✅ 0 💤 0 ❌

Results for commit 7d0a363.

♻️ This comment has been updated with latest results.

[ferdymercury]

Thanks a lot Monica for this alternative suggestion! If we decide to go this way, could you also implement the helper function:

/** copy-doxygen-docu from other pull request, too*/
         template <class Vector>
         Vector Rotate(const Vector & v, double alpha, const Vector & axis) {
            Vector a = v;
            ROOT::Math::VectorUtil::Rotate(a, ROOT::Math::Rotation3D(ROOT::Math::AxisAngle(axis, alpha)));
            return a;
         }

as well as 'rescue' the TRotation alpha fmod fix, and the fixed typoes in the documentation from the linked PR, that could be then closed ?