retrieve

• MR that adds core.group_files_by_tape_id which parses json output from slk_gfbt.

• Adds core.retrieve that accepts all kinds of input and offers the option to group retrieve calls and to delay them.

• This MR also now runs pytest in .gitlab.ci.yaml. There is a basic structure now for checking for slk commands and some tests would require a test system. Test functions decorated with requires_slk will be skipped if no slk command is available.

• Mocking pandas in docs build, it took ages to build in the dkrz image...