Fix type conversion in simplify

[davide-f]

Closes #552

Changes proposed in this Pull Request

This is a simple fix for issue #552 .
The problem was related to type conversions when using the dataframe.loc = pd.Series
This is general bug fixing; no need to add a release_note for that

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 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.

[pz-max]

Just some questions regarding loose comments otherwise this looks good