Add RCM in XSAR

[agrouaze]

• [x] ground_heading check
• [ ] velocity
• [x] flipping checks
• [x] doc sphinx
• [x] modification xsar.py
• [ ] attributs : satellite_height, height -> reference ....

[yreynaud]

• [x] Concatenate reader's datatree in RcmDataset datatree

[agrouaze]

@yreynaud at least one of the notebooks has been pushed with a kernel name that is not the classical "ipykernel (python3)"