[REF] Changes data directory locator for abagen data

[rmarkello]

Closes #44.

Adapts functionality from nilearn to accept a more liberal data_dir specification in abagen.datasets.fetch_microarray(). If data_dir is not provided the function now defaults to storing datasets in $HOME/abagen-data.

[coveralls]

Pull Request Test Coverage Report for Build 246

• 47 of 57 (82.46%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 95.02%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
abagen/datasets.py	24	34	70.59%

Totals
Change from base Build 244:	-0.8%
Covered Lines:	973
Relevant Lines:	1024

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💛 - Coveralls