Invalidate Datasets

Policies concerning invalidation of datasets and their parents. Mainly to circumvent problems with user output registration in DBS.

1. Whenever a dataset is marked INVALID in DBS for a problem in the content, all children should be marked INVALID as well.
2. Whenever a dataset is marked as INVALID or DELETED in DBS because files were list, but the data was originally good, usually we might want to keep the children. In this case we have two options:
   1. Workaround for the current situation: delete/invalidate the parent in PhEDEx (TMDB), but keep the parent VALID in DBS even with 0 replicas (attention, automatic checks have to be adapted!)
   2. Possible future change (cleaner solution?): delete/invalidate the parent in PhEDEx (TMDB), mark the parent with an appropriate state in DBS (e.g. VALID-DELETED), change DBS so that one can register datasets with a VALID-DELETED dataset in their parentage. Not clear if this can be done easily by the DBS devs.