BUGFIX: calculate bootstraps

Bug

Error description

Bootstrap skill scores are ploted for a single combination of method and type only. But all data is calculated!

Error message

First guess on error origin

Error origin

When exception FileNotFoundError was thrown in post_processing.py:PostProcessing:bootstrap_postprocessing, a 2nd iteration is intended to calculate bootstrap with parameter create_new_bootstraps=True. But the 1st line in the method resets the self.bootstrap_skill_scores which leads to a generall reset for all skill scores that have been calculated before.

Solution

Reset is only valid for _iter==0 but not for other recursions.