Merge requests

I cannot create a MR from my fork (the button does not appear). Would like to create this one:

WIP: Index function first occurrence

This branch was tested by merging with branch `first_autumn_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 first_autumn_frost is an annual index (not a seasonal one) (#95)

When the associated issues have been resolved, then the branch first_autumn_frost should go to a MR.