Period.sorting_options not working as expected

[EsGeh]

Sorting Occurrences in combination with caching of Occurrences leads to buggy behaviour.
The sorting_options constructor argument for Period doesn't work as expected i.e. Occurrences in most cases are not sorted corresponding to the sorting_options given.
Reviewing the code I figured this is probably due to a bug excluding cached Occurrences from being sorted.

I created a pull request which aims at fixing this bug:
#516