fix(processor): fix a bug relative to nested list

When a list with a nested list was executed, the rich progress was stopped when the resource stack of the nested list was exiting.

Now when a list is appended to another list, the inner one is flagged as 'nested_list`. This boolean flag is then used in the resource acquisition method to avoid including the progress object in the resource stack.

Similarly to the database connection manager, only the outer list is responsible to clean up resources when finishing.

Fix a missing + 1 in the ProcessorList task incrementation.

Refactor the way the tasks are updated and introduced a minor change in the way the task visibility is defined.

Include new and modify existing tests to cope with the changes.

Changelog: fixed

Signed-off-by: BULGHERONI Antonio antonio.bulgheroni@ec.europa.eu

Closes #61 (closed)