Change reader and component names from "VII" to "METimage" and fix minor issues

[ameraner]

This PR

• changes the name of the METimage instrument reader from vii_l1b_nc and vii_l2_nc to metimage_l1b_nc and metimage_l2_nc. The old reader names are put in deprecation (they still work but emit a FutureWarning). The python files and class names are adapted accordingly.
• The platform name is translated to the OSCAR names (Metop-SG-AX)
• With these fixes, the pyspectral RSR work correctly (from Add metimage average srf pyspectral#278 )

On top, two minor fixes are also implemented:

• the longitude arrays are wrapped to the -180, 180 range to avoid failures in resampling corner cases
• the yaml calibration configuration of one channel was faulty

... and the rows_per_scan attribute is added to datasets to support EWA resampling.

• Closes Rename VII readers and update VII readers for new test data #3202
• Tests added
• Fully documented

FYI @cesclc @TAlonglong @ludwigVonKoopa