Fix rankfilter

Hi,
Here are some fixes for Issue #559 :+1: 
- added some test code, in particular to check if percentile p0=0 must be identical to rank.minimum and percentile p0=1 must be identical to rank.maximum
- fix a bug in percentile in ``_crank16_percentiles.kernel_percentile`` and ``_crank8_percentiles.kernel_percentile`` where the case ``p0=0`` was not properly addressed
- add and example ``plot_percentile.py``similar to this proposed by @RONNCC