Introduces customisable capacity_threshold for MASTR dataset

[jensch-dlr]

Closes #268.

Changes proposed in this Pull Request

• adds a customisable "capacity_threshold" to the config instead of the hard-coded "THRESHOLD_KW" for the MASTR() data importer
• removes possibly misleading comment regarding which capacities are (not) filtered out from "fully_included_sources: MASTR"

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release_notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license and have added my name to the docs/contributors.md.