Write hdf5 data

[stefsmeets]

For compatibility with duqtools, we need a layer to write back changes to the hdf5 file.

[stefsmeets]

I'm thinking this could look something like this:

def to_imas(file: str | Path, arr: xr.Dataset, ids: str, variables: Collection[str]):
    ...

From the variables we can figure out the key where to store the data in the hdf5 file (reverse _var_path_to_hdf5_key_and_slices).