Add operational reserves margins

[pz-max]

Closes #321

Changes proposed in this Pull Request

This PR adds operational reserve margins from PyPSA-Eur as described here: #395

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[pz-max]

@fneum @FabianHofmann thanks for the awesome work. Just to track some changes compared to your changes:

• I add configtables for the new options
• I set the default of contingency to 0 as the contingency is highly dependent on the region of interest e.g. 4000 MW contingency is too high for modelling Croatia only.
• I activate the operational reserve margin in the CI run so that it will be tested

Maybe some of it can be reused in PyPSA-Eur?

[pz-max]

@davide-f I changed the main branch to be protected, meaning we need at least one review before merging.

[davide-f]

Great job! :D