[GitHub-Action]: Auto-formatting based on autoyapf

[jonasjucker]

The Action introduced is as follows:

• Every push to spack-c2sm that includes a *.py files triggers action
• All Python files (not only those contained in commit) in spack-c2sm are checked
• Action creates a new commit in same branch that brings all changes for Pep8-formatting
• Action is disabled for branch master, because we don't want code changes after a merge into master

The first time the action runs, all existing files are formatted. This was done in another branch.
See #298 for the formatting changes.

[jonasjucker]

Pinging @cosunae @lxavier @elsagermann @PanicSheep @mjaehn @leclairm because this PR will modify ALL packages and Python-scripts in spack-c2sm.

Please check if modifications (only formatting) from PR #298 break anything?

[dominichofer]

Automatic pep8 formating is a well established technology. So I don't expect any breaking changes.
LGTM