Expose Occurrences API

[niccokunzmann]

recurring-ical-events works internally with occurrences of components and generates components as an output.

For some purposes, it would be nice to expose this API:

• easy identification of occurrences
• custom generation of components
• architecture: expose the hidden (_method) API in a consistent way
• extension: pagination at the moment requires the private API. Other libraries (such as automations) might want to work with occurrences instead.

See also:

• https://github.com/Frazou1/ical_filter
• https://github.com/Frazou1/ical_filter/issues/1