fix: fix incorrect recognition of the ISO-2 country code "NA" as NaN …

[doneachh]

…value

Closes # (if applicable).

Changes proposed in this Pull Request

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and envs/environment.docs.yaml.
• 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.

[doneachh]

@energyLS i copied the function read_csv_nafix from pypsa-earth _helpers to pypsa-earth sec helpers.
Otherwise i would have update the systempaths for the both scripts and i didnt know whats the more ugly / fancy development strategy.

Alternative:

import sys
sys.path.append('./pypsa-earth/scripts')

in both scripts for the whole script

[energyLS]

Looks fine.