Returns periods and approximate psd values for given percentile value.
Parameters:
percentile (int) percentile for which to return approximate psd
value. (e.g. a value of 50 is equal to the median.)
hist_cum (numpy.ndarray (optional)) if it was already computed beforehand, the normalized
cumulative histogram can be provided here (to avoid computing
it again), otherwise it is computed from the currently stored
histogram.