Re-working time-base interpolation

[rkube]

The current approach of passing a dictionary and magic numbers to define time interpolation is bad design.
Instead we should pass an interpolation object
This could work like this:

tb_ip = causal_interpolator(tstart=0.0, tend=1000.0, deltat=1.0)
my_ds = D3D_dataset(shotnr, tb_ip,
                    predictors=["pinj", "neut", "ae_prob"],
                    targets=["ae_prob_delta"],
                    shift_targets=shift_targets,
                    device=device)

Thus, the new class would eliminate the use of dicts and magic that disable interpolation.