Adding DOUBLE PRECISION to data types, parsing it, and tests

[AugustoFKL]

Adding DOUBLE PRECISION correct support(1) (2).

Since PostgreSQL doesn't accept DOUBLE, we should differentiate it from DOUBLE PRECISION when parsing it. More description is on the related issue (3)

[1] : https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#approximate-numeric-type
[2] : https://www.postgresql.org/docs/current/datatype-numeric.html
[3] : #622

[coveralls]

Pull Request Test Coverage Report for Build 3137733703

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 85.556%

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Totals
Change from base Build 3136407158:	0.009%
Covered Lines:	10087
Relevant Lines:	11790

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💛 - Coveralls