Improve sync field

[sandorkertesz]

Description

This PR:

• improves the syncing of the field and the GRIB handle after setting the high-level metadata and sync() is called
• improves the setting of the raw ecCodes metadata on a Field

In both cases the original data representation of the field is kept. I.e. if the field stores the values in array the updated field will also store the values as an array.

Also improved the way the reference_field of the Xarray accessor is updated when changing it with set().

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.