BUG: segmentation fault when unwrapping 3d image

Unwrapping a 3D image using unwrap_phase no longer causes a segmentation
fault when the wrap_around parameter is True for the middle dimension.
TST: Unit test to check for 3d unwrap seg fault

Added a unit test to check for bug in unwrap_phase which causes
a segmentation fault when the middle dimension is connected.
Merge pull request #1171 from jjhelmus/unwrap_phase_3d_bug

BUG: segmentation fault when unwrapping 3d image