add missing annotation parameters needed for SLC TOPS reading

This PR is meant to prepare the SLC TOPS decoding including the overlapping bursts along azimuth axis in each subswath.

[x] add _number_of_bursts
[x] add _lines_per_burst
[x] add _samples_per_burst
[x] add _azimuth_time_interval
[x] add _npoints_geolocgrid
[x] add height from geolocation grid
[x] add slant_range_time from geolocation grid
[x] add ground_spacing() method in the Sentinel1Dataset()
[x] add informations about swathtiming
[x] validation of the geolocation grid annotations versus cerbere
[x] validation of the swathtiming annotations versus cerbere
[x] valdiation of the doppler centroid estimates annotations versus cerbere
[x] validation of orbit state vector annotations versus cerbere
[x] validation of the azimuth fm rate annotations versus cerbere
[x] validation of the azimuth_time variable versus cerbere
[ ] test EW SLC product
[ ] test IW SLC previous IPF versions
[ ] validate full cross spectrum computation algorithm versus sar_tops_inversion ODL
[x] check that the annotation files is always the right one in sentinel1_meta.py
[ ] fix variables that depend on azimuth_time to skip the overlapping burst lines (8)

umr-lops / xsar