antitarget: set default minimum bin size based on average bin size

If the user specifies a different average bin size, the minimum should change
proportionally. Math: reads per bin ~ bin size; bins of size less than
    avg_bin_size * (2^min_log2_coverage)
are expected to have below the minimum allowable coverage. Double that
size/coverage minimum so we can still potentially detect hemizygous loss:
    min_bin_size = 2 * avg_bin_size * 2^min_log2_coverage)