[ntuple] add read support to SoA I/O

[jblomer]

Part of #19230

Tutorial output:

************************************ NTUPLE ************************************
* N-Tuple : ntpl                                                               *
* Entries : 4                                                                  *
********************************************************************************
* Field 1           : points (PointSoA)                                        *
*   Field 1.1       : _0 (PointRecord)                                         *
*     Field 1.1.1   : fX (float)                                               *
*     Field 1.1.2   : fY (float)                                               *
********************************************************************************
Approximated PI with 100 points to 3.2
Approximated PI with 500 points to 3.104
Approximated PI with 1000 points to 3.172
Approximated PI with 10000 points to 3.1388

[github-actions]

Test Results

    22 files      22 suites   3d 11h 54m 40s ⏱️
 3 863 tests  3 863 ✅ 0 💤 0 ❌
76 218 runs  76 218 ✅ 0 💤 0 ❌

Results for commit 0a58738.

♻️ This comment has been updated with latest results.

[hahnjo]

Very nice! Two minor comments about comments 😉