add s1.meta.azimuth_fmrate

[agrouaze]

address issue #52

• [x] add mappings
• [x] add compound var
• [x] add s1meta property
• [x] validation

[agrouaze]

I can remove the coefficients of the polynomial since they are accessible with s1ds.s1meta.azimuth_fmrate['polynomial'].values[i].coef[::-1].

[agrouaze]

[agrouaze]

API html page after correction