'Constructor.
Args:
period (int): forecast period number
field (string):  field name to use as value
unit_of_measurement(string): unit of measurement'

def __init__(self, period, field, unit_of_measurement=None):

    super().__init__(friendly_name=(lambda wu: wu.data['forecast']['txt_forecast']['forecastday'][period]['title']), feature='forecast', value=(lambda wu: wu.data['forecast']['txt_forecast']['forecastday'][period][field]), entity_picture=(lambda wu: wu.data['forecast']['txt_forecast']['forecastday'][period]['icon_url']), unit_of_measurement=unit_of_measurement, device_state_attributes={'date': (lambda wu: wu.data['forecast']['txt_forecast']['date'])})
