Mixing local files and files from SciCat

[SimonHeybrock]

@wpotrzebowski pointed out that we currently cannot mix files downloaded with pooch with local files. It is all or none.

While we do not plan to support this for pooch (since it is only for testing and documentation), this is an important requirement when working with SciCat. For example, mask files or other references may be obtained from SciCat, while a user wants to process a local file.

[jl-wynen]

For SciCat, I think this is a matter of adding a provider that can turn a dataset id + Filename into a FilePath. This way, all loaders can operate on FilePaths which are then specified either through the SciCat provider or given explicitly as params.