Enhance configuration flexibility

[davide-f]

Closes # (if applicable).

Change proposed in this Pull Request

This PR aims at providing a flexible interface to accommodate custom configuration files.
In particular:

• the docstring of config is added to collect function
• the new extend_config option is added to collect function, whose goal is to update the default config file

Unsure if this is a wanted feature.
I'm thinking that as a workaround, someone may update the config externally, though this is easier for the user.
Currenly, we have this feature in the fork for pypsa-earth.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other

Checklist

• I have added a note to release notes doc/release_notes.rst.
• I have used pre-commit run --all to lint/format/check my contribution
• I have documented the effects of my code changes in the documentation doc/.
• I have adjusted the docstrings in the code appropriately.

[FabianHofmann]

Makes sense :) Can we name the argument config_update?

[davide-f]

@FabianHofmann Done, thanks :)
Sorry for the 3 PRs opened at the same time; they are old changes and I guesssed you preferred to have the features splitted to ease the review.

[FabianHofmann]

No worry, that's good! Makes life easier :)