Add per-dataset and overall summary rows to benchmark results table (#4361)

## Summary
- New `aggregate_scene_metrics` helper computes `__avg__` (mean of
per-scene metrics) and `__all__` (recomputed from pooled raw errors);
applied at category, dataset, and overall levels.
- `Metrics` now carries `errors` and `position_accuracy_gt` so
higher-level pooled aggregates can be reconstructed.
- `create_result_table` emits a `###dataset###` block per dataset with
>1 category and a final `###overall###` block when there's >1 dataset.
- Side fix: per-category `__all__` rows previously reported dataset-wide
image/component totals; now correctly per-category.

Looks like this now:
```
======scenes======= ======AUC @ X deg (%)====== ===images=== =components=
                     0.5    1.0    5.0    10.0     reg   all  num largest

=============================imc2023=haiper==============================
bike                  4.17  22.66  62.63  68.46     15    15    1      15
chairs               12.27  43.38  88.45  94.23     16    16    1      16
fountain             57.63  78.59  95.72  97.86     23    23    1      23
-------------------------------------------------------------------------
overall              34.40  57.38  86.60  90.47     54    54    3      54
average              24.69  48.21  82.27  86.85     18    18    1      18

==============================imc2023=urban==============================
kyiv-puppet-theater   1.16   6.50  31.65  43.52     25    26    1      25
-------------------------------------------------------------------------
overall               1.16   6.50  31.65  43.52     25    26    1      25
average               1.16   6.50  31.65  43.52     25    26    1      25

============================imc2023=heritage=============================
cyprus               44.92  68.51  88.37  90.85     29    30    1      29
dioscuri             45.54  63.51  87.73  92.71    173   174    1     173
wall                 50.38  74.43  94.89  97.44     43    43    1      43
-------------------------------------------------------------------------
overall              45.79  64.24  88.14  92.92    245   247    3     245
average              46.95  68.82  90.33  93.67     82    82    1      82

==========================imc2023=phototourism===========================
piazza_san_marco     15.17  39.62  80.60  89.39     68    68    1      68
sagrada_familia      22.94  37.94  59.00  62.95     90    90    1      90
st_pauls_cathedral   21.35  41.90  74.29  82.00    142   142    1     142
-------------------------------------------------------------------------
overall              20.87  40.61  71.41  78.35    300   300    3     300
average              19.82  39.82  71.30  78.11    100   100    1     100

#################################imc2023#################################
overall              33.07  52.08  79.43  85.31    624   627   10     624
average              27.55  47.70  76.33  81.94     62    63    1      62

===============================eth3d=dslr================================
door                 24.19  46.95  84.62  92.55      7     7    1       7
lounge               21.20  27.26  32.12  32.73      6    10    1       6
statue               94.47  97.23  99.45  99.72     11    11    1      11
-------------------------------------------------------------------------
overall              54.86  62.34  71.81  73.55     24    28    3      24
average              46.62  57.15  72.06  75.00      8     9    1       8

================================eth3d=rig================================
storage_room          7.49  13.42  21.97  23.43    398   796    1     398
storage_room_2        1.68   5.09  17.30  20.75    416   832    1     416
terrains              0.86   4.40  18.20  21.31    330   660    1     330
-------------------------------------------------------------------------
overall               3.57   7.91  19.21  21.85   1144  2288    3    1144
average               3.34   7.64  19.16  21.83    381   763    1     381

##################################eth3d##################################
overall               3.57   7.92  19.21  21.86   1168  2316    6    1168
average              24.98  32.39  45.61  48.41    195   386    1     195

#################################overall#################################
overall               4.65   9.54  21.42  24.19   1792  2943   16    1792
average              26.59  41.96  64.81  69.37    112   184    1     112
```