Rename LearnerCrossfit.n_fits_ to LearnerCrossfit.n_splits_

[j-ittner]

This PR renames LearnerCrossfit.n_fits_ to LearnerCrossfit.n_splits_ for more consistent terminology: we refer to the elements of a crossfit as the splits not the fits.

[joerg-schneider]

@j-ittner
Should n_fits terminolgy remain here?:
https://github.com/BCG-Gamma/facet/blob/984e9cc94adc52a6d9c1181a525e95f135326cb7/src/facet/crossfit/_crossfit.py#L272-L274

Thinking of resize, there seem to be two concepts we call the same

1. initially done fits/splits as defined via underlying CV
2. projection to a subset of 1) to be selected for the new crossfit object – maybe calling the function argument to_n_splits or keep_n_splits makes that easier to grasp from the outside

[j-ittner]

@j-ittner
Should n_fits terminolgy remain here?:

https://github.com/BCG-Gamma/facet/blob/984e9cc94adc52a6d9c1181a525e95f135326cb7/src/facet/crossfit/_crossfit.py#L272-L274

Thinking of resize, there seem to be two concepts we call the same

1. initially done fits/splits as defined via underlying CV
2. projection to a subset of 1) to be selected for the new crossfit object – maybe calling the function argument to_n_splits or keep_n_splits makes that easier to grasp from the outside

I should change this, well spotted!