Allow constant domain instead of vector in `sample_domain`

[rflamary]

While this feature makes no sens for fitting, it can be very practical for multi-source/target for instance it would allow to sometging like

pipe = make_da_pipeline(
     PerDomain(StandardScaler()),
     SelectSource(SVC()), 
)

pipe.fit(X, y, sample_domain=sample_domain)

pipe.predic(Xs,sample_domain=1)
pipe.predic(Xt,sample_domain=-1)