Computes the minimum number of tags required in window for an island to be enriched.

def compute_enriched_threshold(average_window_readcount):
    # type: (float) -> int
    """
    Computes the minimum number of tags required in window for an island to be enriched.
    """

    current_threshold, survival_function = 0, 1
    for current_threshold in count(start=0, step=1):
        survival_function -= poisson.pmf(current_threshold,
                                         average_window_readcount)
        if survival_function <= WINDOW_P_VALUE:
            break

    island_enriched_threshold = current_threshold + 1

    return island_enriched_threshold