Change PerformanceEvaluation display layout

[ablaom]

Resolves #1070 by moving the uncertainties (1.96*SE) in display of PerformanceEvaluation objects. After this PR:

PerformanceEvaluation object with these fields:
  model, tag, measure, operation,
  measurement (per-fold aggregate), uncertainty_radius_95 (1.96*SE),
  per_fold, per_observation,
  fitted_params_per_fold, report_per_fold,
  train_test_rows, resampling, repeats
Tag: tag
Extract:
┌───┬──────────────────────────────┬──────────────┬─────────────┬─────────┐
│   │ measure                      │ operation    │ measurement │ 1.96*SE │
├───┼──────────────────────────────┼──────────────┼─────────────┼─────────┤
│ A │ bogus                        │ predict      │ [1.0]       │         │
│ B │ MulticlassFScore(            │ predict_mode │ 0.286       │ 0.0     │
│   │   beta = 1.0,                │              │             │         │
│   │   average = MacroAvg(),      │              │             │         │
│   │   return_type = LittleDict,  │              │             │         │
│   │   levels = nothing,          │              │             │         │
│   │   perm = nothing,            │              │             │         │
│   │   rev = nothing,             │              │             │         │
│   │   checks = true)             │              │             │         │
└───┴──────────────────────────────┴──────────────┴─────────────┴─────────┘
┌───┬────────────────┐
│   │ per_fold       │
├───┼────────────────┤
│ A │ [[1.0], [1.0]] │
│ B │ [0.286, 0.286] │
└───┴────────────────┘
Apply `describe` to this result for a named tuple summary.