Custom ppl opts

[yerbol-akhmetov]

Changes proposed in this Pull Request

Here I propose an extension of functionality of custom powerplants addition. Currently, there are only two options: 1) Use open-source powerplants with custom_powerplants: false, and 2) Attach custom powerplants from custom_powerplants.csv to open-source data and use both with custom_powerplants: true. There is no option to select only custom powerplants. This functionality is very useful. Sometimes you can have an accurate custom powerplants information and you would like to replace retrieved open-source plants with custom data. On the other situations, you might need to use both open-source and custom powerplants.
Therefore, I propose to extend the functionality to three options:

1. false - no custom power plants are used;
2. merge - both open-source and custom powerplants from custom_powerplants.csv are merged and used;
3. replace - only custom powerplants from custom_powerplants.csv are used.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.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]

Nice PR! 💯 Some minor changes are required.
Could you also adjust the configuration.rst accordingly?

[pz-max]

Could you change the comment to wherever possible:
# "false" use only powerplantmatching (ppm) data, "merge" combines ppm and custom powerplants, "replace" use only custom powerplants

[pz-max]

I know pypsa-eur is using line 247 and 248. Can you figure out its purpose?
I think your merge and replace option could make lines 247 and 248 not necessary. Maybe we can remove the comment?

[yerbol-akhmetov]

I have checked the code and it seems that it is left to enable some query to custom powerplants being added. We can leave them as commented or we can remove them.

[yerbol-akhmetov]

But it is better to remove them, since if enabled it might contradict with the proposed PR.

[pz-max]

Looks great. I just had to change the release note position