Feature: Added environment override for dipy_home variable

[tomwright01]

The fetcher module defines a path based from the users home directory. This can cause a problem when running in a cluster environment where the home directory can be machine specific.

This PR enables an environment variable DIPY_HOME to override the default setting.

[coveralls]

Coverage decreased (-0.003%) to 88.591% when pulling 8eaf635 on tomwright01:master into 379ea81 on nipy:master.

[codecov-io]

Codecov Report

Merging #1233 into master will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff            @@
##           master   #1233      +/-   ##
=========================================
- Coverage    86.1%   86.1%   -0.01%     
=========================================
  Files         223     223              
  Lines       27893   27895       +2     
  Branches     2828    2829       +1     
=========================================
  Hits        24018   24018              
- Misses       3179    3180       +1     
- Partials      696     697       +1

Impacted Files	Coverage Δ
dipy/data/fetcher.py	40.11% <33.33%> (-0.24%)	⬇️

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[coveralls]

Coverage decreased (-0.003%) to 88.591% when pulling b4488d2 on tomwright01:master into 379ea81 on nipy:master.

[coveralls]

Coverage decreased (-0.003%) to 88.591% when pulling efdb9d0 on tomwright01:master into 379ea81 on nipy:master.

[arokem]

This looks fine to me. I've heard people complain that we are not using and XDG-compliant directory for the data, but as long as we are going to do this, I am fine with adding this additional twist. Anyone else have any comments about this?

[MarcCote]

Good idea. LGTM