Gbasf2: Fix moving of downloaded multi-sub datasets

[meliache]

For datasets with multiple subs, when moving the contents of the temporary project download directory to the final ouptut dir, previously, there was an error which led to the file structure:

<result_dir>/B.root/job_name*B.root  # sub00 contents
<result_dir>/B.root/sub01/job_name*B.root
<result_dir>/B.root/sub02/job_name*B.root

This commit should fix this back to

<result_dir>/B.root/job_name*B.root  # contents of all subs

Further, since this moving function turns out to aggregate many lines of code, I moved it into an own helper function, which can be independently tested.

Draft so that I can link to it, but I haven't finished yet writing unit tests.

• [x] Add unit-tests for multi-sub directory structures with dummy root files
• [x] Run a job on the grid with one input file to check that the single-sub case still works.

Resolves #147

[meliache]

Added some unittests on dummy directory strucutres and now this should be ready. The only problem I have currently is that when I run test-jobs on the grid I get download failures, but which according to the mailinglist seem to be caused by problems on the SE sites, so I can't test at the moment.