add climate indices from psl noaa

[aaronspring]

Adds:

• climate indices in csv format from psl.noaa.gov
• helper function to convert these to xr.DataArrays

Can be extended:

• creating subcatalog with more indices from psl.noaa
• many correlations kicked out for data formatting reasons
• timeseries: https://www.psl.noaa.gov/data/timeseries/monthly/

import pandas as pd
indices = ['pna','wp','ea','nao','jonesnao','soi','nina3','censo','oni','nina1','nina4','nina34','np','hurr','pacwarm','swmonsoon']
fig,ax=plt.subplots(nrows=len(indices),figsize=(8,20),sharex=True)
for i,index in enumerate(indices):
    url = f'https://psl.noaa.gov/data/correlation/{index}.data'
    print(index, url) 
    df = cat.climate.indices(index=index).read()
    da = monthly_csv_to_DataArray(df)
    da.plot(ax=ax[i])
    ax[i].set_title(f'{index.upper()} from {url}')
plt.tight_layout()