Add load world population data

[ekatef]

Closes #18

Changes proposed in this Pull Request

Added functionality to download world population data: mainly copy-pasted from pypsa-earth with some minor adjustments. Only a approach basic approach is currently implemented: to load data on 1x1km raster, although using of fine resolution (100x100 m) can be probably also of interest.

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [x] Code and workflow changes are sufficiently documented.
• [ ] Newly introduced dependencies are added to the main environment at PyPSA-Earth repository
• [ ] Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• [ ] Add a test config or line additions to test/ (note tests are changing the 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.

[ekatef]

As a double check, would be nice to understand a purpose of size_min argument. As far as I understand, it is needed as a sanity check for the loaded data, but I'm not quite sure in which exactly cases it's needed.