enh(add_existing_baseyear): Pull out valid_grouping_years algorithm from heat

[coroa]

Changes proposed in this Pull Request

Minimal refactoring to re-use the grouping_years validation also for power. And fix a wrong indent.

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in config/config.default.yaml.
• Changes in configuration options are documented in doc/configtables/*.csv.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.

[FabianHofmann]

I am a bit confused about this expression here, this can be a negative number or? If grouping_year[0] == 1990 and baseyear == 2020 and lifetime == 20 for example.

[coroa]

I must admit, i did not think about this. It is a literal port of Amos':

# Fill NA from .diff() with value for the first interval
_years[0] = valid_grouping_years[0] - baseyear + default_lifetime

[coroa]

Ok, sorry, Amos, i was wrongly blaming you. The origin story of that expression is complicated.

@lisazeyen came up with:
https://github.com/PyPSA/pypsa-eur/blame/a72388b989d1d667ec7e44d66f6c3b494b46d000/scripts/add_existing_baseyear.py#L466-L467

         _years = (valid_grouping_years.diff().shift(-1)
                  .fillna(baseyear-valid_grouping_years.iloc[-1]))

ie. (baseyear - last of grouping_years) which sounds quite sensible (for an interval, unless the last grouping year is the same as the baseyear).

and then @lindnemi changed it to:

# Fill NA from .diff() with value for the first interval
_years[0] = valid_grouping_years[0] - baseyear + default_lifetime

in #1102 with associated discussion in #1091 .

[coroa]

It sounds like there was some back and forth, i'll not try to understand this fully now, maybe its best to have a short discussion.