bugfix: handle demand edge cases

[daniel-rdt]

Changes proposed in this Pull Request

This PR fixes a bug to handle TYNDP electricity demand edge cases for NT 2050, and intermediate years between 2030, 2040 and 2050.
The PR also adds 2035 to config/test/config.tyndp.yaml to test for intermediate year in future.

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.
• Changes in configuration options are added in config/test/*.yaml.
• Sources of newly added data are documented in doc/data_sources.rst.
• A release note doc/release_notes.rst is added.