Rename index function percentile_occurrence for consistency

In setup.py we now (tag 0.13) have the line

'percentile_occurrence=' 'climix.index_functions:PercentileOccurrence',

However, as the index function really counts the number of occurrences the name should be changed to reflect this. Such a change is consistent with existing names. Therefor the index function name should be changed to:

'count_percentile_occurrences=' 'climix.index_functions:CountPercentileOccurrences',