Provide GCPs or at least extent with burst data

[alexamici]

At the moment the only way to link the burst to the associated GCPs is to match the azimuth_times and slant_range_times.

It would be helpful to attach some information on localisation of the burst (computed to define the burst name) to the burst dataset.

[alexamici]

Added the get_footprint_linestring and the gcp arguments to crop_burst_dataset.

This is very tricky because interpolating on the numerically very different azimuth_time and slant_range_time is unstable and interpolating on line and pixel doesn't return the external polygon, but a slightly shorted one along the azimuth due to how crazily the bursts are encoded in a single tiff file.

This is good enough for many use cases.

[alexamici]

In the end the it is closed by #115