Add dimension separator as a type parameter

[mkitti]

This replaces #150 by implementing the dimension_separator as a type
parameter.

I also intend to add Zarr specification version as a type parameter in forthcoming commits.

[mkitti]

cc: @bjarthur @StephanPreibisch

[coveralls]

Pull Request Test Coverage Report for Build 13829000977

Details

• 38 of 51 (74.51%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 85.189%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Storage/dictstore.jl	2	3	66.67%
src/Storage/directorystore.jl	3	4	75.0%
src/Storage/gcstore.jl	2	3	66.67%
src/Storage/http.jl	2	3	66.67%
src/Storage/zipstore.jl	2	3	66.67%
src/ZArray.jl	2	3	66.67%
src/Storage/consolidated.jl	5	7	71.43%
src/metadata.jl	9	11	81.82%
src/Storage/s3store.jl	3	6	50.0%

Totals
Change from base Build 13680422725:	-0.8%
Covered Lines:	926
Relevant Lines:	1087

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💛 - Coveralls