Make a fast-track L2 product before any derived variables are being calculated

[BaptisteVandecrux]

• stored in a separate folder
• can be used for the BUFR files processing

[PennyHow]

• [ ] Write out L2 product in a new get_l2 CLI script
• [ ] Move data file merging to after L2
• [ ] Implement flags for hard merging, or combine_first