`accuracy()` in wide form

The output of `accuracy()` in wide form is actually the correct way to store, I think. Bc the metrics like `RMSE` and `MAPE` have different units, which are better to separate them in columns. If organising in long form like `.metric` and `.estimate`, it would lead to the thinking that they share the same units.

I wouldn't address something like model-centric or data-centric.