Add rejection sampling capability

[sdl1]

It would be useful to be able to select the seed for (e.g.) the RandomSelector based on user-specified criteria. This would allow an implementation of more sophisticated sampling methods (cf. #7 ) via rejection sampling, with a compact representation (the seed) of the final date set. Something like the following:

function acceptance(validation_dates, holdout_dates)
    # We want 1st of January to be in the validation dates
    return DateTime(2021, 1, 1) in validation_dates
end

# Gives a seed with the desired properties to use with RandomSelector
seed = rejection_sample(RandomSelector, date_range, acceptance)