Add gradient data write functionality

[davidscn]

... in case gradient data is required for the mapping.

The current changes only implement the functionality for a simple CHT case. We need to go through all relevant fields and have a look, where it makes sense to support this feature. We also need to find a proper solution in order to avoid code duplication in all the fields, as the 'write' step looks similar for all fields (mainly depending on the number of data components).

TODO list:

• [ ] I updated the documentation in docs/
• [ ] I added a changelog entry in changelog-entries/ (create directory if missing)