Support float seasonal_periods in Prophet

[gregorp90]

Is your feature request related to a current problem? Please describe.
I would like to add a custom seasonality to the Prophet model. For example, weekly data with yearly seasonality. This set up requires 52.18 periods. But Darts requires the seasonal_periods to be an integer and raises an exception if a float is provided, while base Prophet allows floats.

At the moment we can only use 52, which will shift the seasonality if we are forecasting over a longer period, which is wrong.

Describe proposed solution
Change the seasonality_properties dictionary in _store_add_seasonality_call in the Prophet model so that seasonal_periods can be a float.

[madtoinou]

Fixed by #1829.