Validate uniqueness of _aggregator_identifier

[cessda-bitbucket-importer]

Original report on BitBucket by Toni Sissala (GitHub: toni-sissala).

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Validate uniqueness of \_aggregator\_identifier in mongodb collection.

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[cessda-bitbucket-importer]

Original comment by Toni Sissala (GitHub: toni-sissala).

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Merged in feature/13-validate-aggregator-identifier (pull request #21)

Add MDB validation and indexing of _aggregator_identifier

Study._aggregator_identifier must be unique within the collection in MongoDB. Add it as an unique index to MongoDB.

Change MongoDB index sort order of Study.study_number from descending to ascending.

Use argparse.RawDescriptionHelpFormatter to disable formatting of module level docstring when outputting db_admin.py help.

Write missing docstring comments to db_admin.py.

Use enum validation items in validation schema at
CDCAggDatabase._prepare_validation_schema().

Fix faulty logic in TestCaseBase.get_app() where the database is
instantiated from Kuha Document Store database module. The correct way
is to instantiate the cdcagg_docstore.controller database
CDCAggDatabase.

Add added-item to 0.2.0 changelog entry.

Fixes #13 at Bitbucket