Added masking on shapefile using salem

[mirkodandrea]

I've implemented an example of dataset masking on a shapefile using the salem library

The code is included at the end of the notebook

import salem

ds = xr.open_dataset(ds_name)
# Load the shapefile
shp_df = salem.read_shapefile(shapefile)
# Query for the US
shp_df_us = shdf.query('CNTRY_NAME=="United States"')
# mask the dataset on the selected geometries
ds_us = ds.salem.roi(shape=shp_df_us)