implement `train_test_split='fair-sliding'/'fair-all'`

[aaronspring]

Is your feature request related to a problem? Please describe.
New paper Risbey et al. 2021 demonstrates how forecaster choices for time selection in bias reduction impacts skill.

Reference: https://t.co/HE4PoROb8Z?amp=1

Describe the solution you'd like
hindcast.remove_bias(how='additive_mean', alignment='same_verifs', method='fair_sliding')

implemented method from:

• unfair implement train_test_split keyword #655
• unfair-cv implement train_test_split keyword #655
• fair implement train_test_split keyword #655
• fair-sliding
• fair-all

unfair-cv is often standard, see #643, but unfair wrt real-time forecast/attainable skill, i.e. unfair means artificial skill

Demo: https://gist.github.com/aaronspring/94e24fb7e0a536be61caf1970287f3b5

[aaronspring]

API: method

• fair: train_init=slice(start, end) ,test_init=slice(start, end) (need to change maybe to train_time for same_verifs)
• fair-sliding: train_init=slice(start, end),test_init=slice(start, end)
• fair-all: test_init=slice(start, end) ,train_init=slice(start, end)
• unfair-cv, cv='loo'
• unfair: now default