Json merge

[doutriaux1]

@lee1043 this one is for you it allows to merge multiple level of the json_structure into a single dimension.
@durack1 @gleckler1 please review as well.
@zshaheen I wouldn't mind you taking a look either to double check all my crazy transpose and recursive functions.
We will need this for the click_portrait.

[doutriaux1]

@lee1043 look at tests to see how I join multiple dimensions together.

[coveralls]

Pull Request Test Coverage Report for Build 749

• 139 of 143 (97.2%) changed or added relevant lines in 2 files are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 74.492%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pcmdi_metrics/io/base.py	100	104	96.15%

Files with Coverage Reduction	New Missed Lines	%
pcmdi_metrics/io/base.py	1	90.0%
tests/basepmpdriver.py	1	90.63%
tests/basepmp.py	15	55.32%

Totals
Change from base Build 743:	0.3%
Covered Lines:	2935
Relevant Lines:	3940

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💛 - Coveralls

[doutriaux1]

@lee1043 @gleckler1 @durack1 ready for review