Update li_l2-reader to read MTG LI L2 test data

[sjoro]

Feature Request

Is your feature request related to a problem? Please describe.
The current li_l2-reader does not support reading of the EUMETSAT released LI L2 test data.
https://www.eumetsat.int/new-li-test-data-released-2021

Describe the solution you'd like
Update the reader to support the latest test data.

Describe any changes to existing user workflow
None

[sjoro]

EUM has a project planned to carry out this update, delivery could be expected in Jan 2023. In the meanwhile we look into a possibility to make a draft-PR shortly with the reader we already have in house (in order to allow users to explore the test data already now). The reader will need a some refactoring + fixing the unittests accordingly.