Add S2/HLS snow cover

[egagli]

Description

Would be nice to add built in snow cover methods beyond NDSI to the Sentinel-2 and/or HLS class.

Potential data sources:

let-it-snow

Theia Snow collection: high-resolution operational snow cover maps from Sentinel-2 and Landsat-8 data

A Revised Snow Cover Algorithm to Improve Discrimination between Snow and Clouds: A Case Study in Gran Paradiso National Park

Evaluation of a High-Resolution Operational Snow Cover Area Classification Algorithm

https://github.com/orfeotoolbox/let-it-snow/tree/master

https://github.com/chiararik/rLIS_VLab

Example

Would be nice to have something like...

s2 = easysnowdata.remote_sensing.Sentinel2(
    bbox_input=bbox,
    start_date="2022-07-21",
    end_date="2022-07-31",
    resolution=80,
)

s2.get_snow_cover()

s2.snow_cover