Fix discovery of array columns

[agy-why]

fixing the metadata for array columns such that discovery and votable export works

[agy-why]

Do not forget to test CSV and FITS exporter

[coveralls]

Pull Request Test Coverage Report for Build 5726333616

• 2 of 7 (28.57%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 64.046%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
daiquiri/core/generators.py	0	5	0.0%
<!--	Total:	2	7	28.57%	-->

Totals
Change from base Build 5715994248:	-0.04%
Covered Lines:	4717
Relevant Lines:	7365

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💛 - Coveralls