Clarify resolution of relative links used by multi-level datasets

[forman]

Is your feature request related to a problem? Please describe.

Following the xcube Multi-Resolution Datasets Specification, relative links should be relative to a multi-level dataset's top-level directory. This is not yet consitently implemented in xcube:

1. if such datasets are opened using the filesystem data stores, such as "file" or "s3", relative links are resolved against the data store's root directory.
2. in other places either relative links don't work at all ("s3" paths) or are resolved against the current working directory.

Describe the solution you'd like

Resolve relative links consitently against the multi-level dataset's top-level directory.

EDIT

In the longer term, filesystem data stores should be the only way open and write any datasets, see #516.

[forman]

Closed by #637