harvester: refactoring and versioning harvester

[ChiaraBi]

• refactoring (close #58)
• Zenodo versioning harvester (close #60)

[coveralls]

Pull Request Test Coverage Report for Build 201

• 44 of 185 (23.78%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-3.4%) to 71.603%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asclepias_broker/harvester/tasks.py	2	10	20.0%
asclepias_broker/harvester/zenodo.py	20	79	25.32%
asclepias_broker/harvester/metadata.py	10	84	11.9%
<!--	Total:	44	185	23.78%	-->

Files with Coverage Reduction	New Missed Lines	%
asclepias_broker/harvester/metadata.py	2	23.31%
<!--	Total:	2		-->

Totals
Change from base Build 192:	-3.4%
Covered Lines:	1175
Relevant Lines:	1641

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💛 - Coveralls