REBASE: Resolve first conflict
ENH: `spacing` kwarg for random_walker and improved tests
FIX: roll back incorrect testing change of gradient order
DOC: Add removal of `depth` from random_walker in 0.10
DOC: Fix minor typo in docstring
Merge pull request #775 from JDWarner/spacing_random_walker

Add generalized anisotropic spacing support to random_walker