Add index function last_occurrence (closes #97)
This branch was tested by merging with branch `last_spring_frost`. The following issues remains:
* [ ] create and output proper metadata to distinguish between missing data and no occurrence (#93)
* [ ] day-of-year is counted from beginning of the season, not beginning of the year (#94)
* [ ] the last_autumn_frost is an annual index (not a seasonal one) (#95)
Edited by Klaus Zimmermann