RF: refactor dpy io tests to fix windows error

Try to fix windows access error by using InTemporaryDirectory rather
than a temporary file.
TST: Reduce the required precision here.

So that WIN32 can rejoin civilized society.
Merge pull request #1 from arokem/mbrett-windows-test-fixes

TST: Reduce the required precision for test.
RF: use sequences for run_command script tester

`run_command` was using strings rather than sequences passing in the
``Popen`` constructor.  This blows up when there are space in the passed
paths.  Use the sequence command definition instead.
RF: put pwd on path running tests from dipy dir

Some of the script tests change directory before running the scripts.
These will fail if the dipy we picked up is in the current working
directory (running from the dipy root directory). Check for this case
and put the directory on the path before running the scripts.
BF: ensure ravel_multi_index has good working type

Indices could overflow if input type of index array was not large
enough to hold large indices.
Merge pull request #309 from matthew-brett/windows-test-fixes

MRG: Windows test fixes