Add a general level1c runner to the repo

[adybbroe]

Code Sample, a minimal, complete, and verifiable piece of code

# Your code here

Problem description

Add a general (supporting any imager that the https://github.com/foua-pps/level1c4pps supports) level1c runner to this repo.

There is a seviri-level1c runner already, but add this code here, as it is anyhow needed for future PPS versions (version 2021 and onwards).

Expected Output

Actual Result, Traceback if applicable

Versions of Python, package at hand and relevant dependencies

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