Files remain in queue when opened but not read

[observingClouds]

Some of the current tests load files from the mock-up tape archive but do not read them. As a consequence self._cache_files() is never called and as such self._file_queue.get() and self._file_queue.task_done(). Those files remain in the queue and cause problems in other tests.

test_reading_dataset cannot be currently called after calling test_ro_mode

Opening files in a context-manager is probably a good idea, but we might also want to have a way to reset the queue or only process the part of the queue, that we are actually requesting.

[antarcticrainforest]

I think that should be closed now, am I right?