Feature - add support to retrieve flood stages

Created by: elmerehbi

I propose to add support for retrieving flood stages from the water service.

The following retrieves all flood stages in a single call:

all_flood_stages = get_flood_stages()

To get the flood stages for a particular site number: get_flood_stage(site_no=["07144100"])

For multiple sites:

stations = ["07144100", "07144101"] print(get_flood_stage(stations)) {'07144100': {'action_stage': '20', 'flood_stage': '22', 'moderate_flood_stage': '25', 'major_flood_stage': '26'}, '07144101': None}