s1meta needs access to fmrate annotations

[agrouaze]

Sentinel1 products annotations contains information related to FM (Frequency Modulated) rate: see https://sentinel.esa.int/documents/247904/1877131/Sentinel-1-Product-Specification

These information are useful in chirp de-ramping for instance. data is stored in xml files like this:

azimuthFmRateList count="11">
      <azimuthFmRate>
        <azimuthTime>2021-01-02T16:31:42.597648</azimuthTime>
        <t0>5.674779755092819e-03</t0>
        <azimuthFmRatePolynomial count="3">-2.170515874841713e+03 3.988051193274622e+05 -6.535396014283583e+07</azimuthFmRatePolynomial>
      </azimuthFmRate>
      <azimuthFmRate>

these annotations could be accessible in s1ds.s1meta.azimuth_fmrate object.

[agrouaze]

issue closed with #53