Feature proposal: add low_price_periods

[Pluimvee]

There is a need for simple-to-use low_price_period indicators which we could add as attributes to one of the sensors.

Next to the minimum price period which we have as sensor (reflecting a low_1hr_period) we could add some attributes indicating

• low_2hr_period: off
• low_3hr_period: off
• low_4hr_period: on
• low_5hr_period: on

The attributed would turn 'on' when x-hrs of sequential hours are the lowest in the configured analysis_window and turn 'off' when that period has passed.
It may be expected that the low_5hr_period turns on earlier than the low_2hr_period as the latter would wait for a lower price. But they can also turn on at the same time and differ only in the moment of turn off again.