refactoring of xsar to remove classes or at least to separate I/O from operations

[agrouaze]

Currently we have no unit test because we cannot not ship the classical input products (Sentinel-1, RadarSat-2 or RCM) which are too heavy.
If xsar was refactored to have methods (such as denoising, calibration, generation of the "measurement" node, ...) with xr.Datatree as input and output, we could test those functions easily with dummy datatrees.