Amend index function `count_percentile_occurrences` to produce output either in days or percent
CLIX-META issue #62 points out that there is an inconsistency in definitions of various percentile-based temperature indices between ETCCDI and and ECA&D.
To account for this inconsistency the CLIMIX index function count_percentile_occurrences
should be able to produce results expressed either as % of calculation period (year, month, or any other), or as days according to what is provided from CLIX-META. As neither standard name nor cell method is currently available there is no complications regarding units or otherwise in that respect.
In the CLIX-META issue it is suggested that the decision on output unit could be defined by the user. Whether this should also be possible in CLIMIX should be handled in a different issue.