Add optional detection of seasonal hotspots to hotspot_classify()

[mpjashby]

A seasonal hotspot would be one that is a hotspot for at least hotspot_prop proportion of periods in a given time of year and a hotspot for less than hotspot_prop the rest of the year. This will probably need new parameters that specify the length of a seasonal cycle and the maximum proportion of a cycle that can be considered a single season.

This would only work for data that contain a certain number of seasonal periods.

This issue should be considered low priority because it will be complicated to fix and the benefit is probably relatively small.