Add CODEOWNERS

[lkstrp]

On the way to an improved PR review process, this PR adds a code owner file.
The files listed below are assigned to the developers who have written the most code for them and know them best. All other files have too many different authors, but the ultimate goal would be to assign a code owner to most scripts over time.

Being the code owner simply means, that there is a mark in GitHub for each assigned file. So that the author/responsible person can be identified without having to go through a lot of git blame.
At the same time, the code owner is automatically added as a reviewer to PRs related to that code.
Which means the code owner will be responsible for the first review from now. Small changes can also be merged and for bigger changes, a maintainer will review again and merge the PR.

More informations on Code owners in GitHub can be found here.

For those in the first batch, please give me a thumbs up if you agree!
@amos-schledorn
@LukasFrankenQ
@toniseibold
@Irieo
@p-glaum
@bobbyxng
@lisazeyen
@FabianHofmann

And always feel free to take any other "ownership" 🧙

Click me

scripts/build_central_heating_temperature_profiles              @amos-schledorn
scripts/build_cop_profiles                                      @amos-schledorn
scripts/definitions/heat_sector.py                              @amos-schledorn
scripts/definitions/heat_system_type.py                         @amos-schledorn
scripts/definitions/heat_system.py                              @amos-schledorn

scripts/build_egs_potentials.py                                 @LukasFrankenQ

scripts/retrieve_eurostat_data.py                               @toniseibold

scripts/build_gdp_pop_non_nuts3.py                              @Irieo

scripts/build_line_rating.py                                    @p-glaum
scripts/build_ship_raster.py                                    @p-glaum
scripts/build_transmission_projects.py                          @p-glaum

scripts/build_osm_network.py                                    @bobbyxng
scripts/clean_osm_data.py                                       @bobbyxng
scripts/prepare_osm_network_release.py                          @bobbyxng
scripts/retrieve_osm_data.py                                    @bobbyxng

scripts/build_retro_cost.py                                     @lisazeyen
scripts/build_monthly_prices.py                                 @lisazeyen
scripts/retrieve_jrc_idees.py                                   @lisazeyen
scripts/_benchmark.py                                           @lisazeyen
scripts/prepare_perfect_foresight.py                            @lisazeyen
scripts/make_summary_perfect.py                                 @lisazeyen

scripts/plot_validation_electricity_production.py               @fabianhofmann
scripts/build_electricity_prices.py                             @fabianhofmann
scripts/build_cross_border_flows.py                             @fabianhofmann
scripts/retrieve_electricity_demand.py                          @fabianhofmann
scripts/plot_validation_electricity_prices.py                   @fabianhofmann
scripts/plot_statistics.py                                      @fabianhofmann
scripts/build_electricity_production.py                         @fabianhofmann
rules/validate.smk                                              @fabianhofmann