Data model rework

[aperrin66]

• Replace GCMD vocabularies models by a generic Keyword model
• Add Tag model to be able to add "key: value" tags to Datasets
• Reduce required fields to instantiate a Dataset to the minimum

[coveralls]

Pull Request Test Coverage Report for Build 18780741707

Details

• 77 of 77 (100.0%) changed or added relevant lines in 9 files are covered.
• 33 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-6.5%) to 89.524%

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Files with Coverage Reduction	New Missed Lines	%
geospaas/base_viewer/forms.py	8	50.0%
geospaas/base_viewer/views.py	25	47.92%

Totals
Change from base Build 9219729886:	-6.5%
Covered Lines:	282
Relevant Lines:	315

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💛 - Coveralls