setting slk version to "unknown" causes issues in pyslk.parsers

Problem

When slk does not exist or slk version is not available then the slk version is set to "unknown":

https://gitlab.dkrz.de/hsm-tools/pyslk/-/blob/master/pyslk/__init__.py#L30

This causes issues when the version string is parsed:

https://gitlab.dkrz.de/hsm-tools/pyslk/-/blob/master/pyslk/parsers.py#L26

I.e. we cannot generate the pyslk.parsers API doc due to this:

• https://gitlab.dkrz.de/hsm-tools/pyslk/-/jobs/37874
• https://hsm-tools.gitlab-pages.dkrz.de/pyslk/pyslk_api.html#pyslk-parsers-module

suggested solution

use 0.0.0 instead of unknown when no version is available.

@g300046 What do you think? I can do the modification. Just wondering if there is an alternative solution.