BUG: Use NRRD for the 6D ImageIO instead of MetaImage.

Try to avoid some 'double free or memory corruption' errors.
Merge pull request #322 from thewtex/nrrd6d

BUG: Use NRRD for the 6D ImageIO instead of MetaImage.
ENH: Use REQUIRED_FILES for the metric extractor test.

Avoid false positives on the dashboard when the dependent test does not
successfully complete.
Merge pull request #324 from vovythevov/MiscTubeTKFixes

Misc tube tk fixes
Merge pull request #323 from thewtex/metric-extractor-required-files

ENH: Use REQUIRED_FILES for the metric extractor test.
ENH: SegmentTubes test baseline after bug fixes in RidgeTraversal
Merge pull request #325 from aylward/SegmentTubes

ENH: SegmentTubes test baseline after bug fixes in RidgeTraversal
ENH: Improved estimation of SegmentTubes parameters
Merge pull request #326 from aylward/SegmentTubes

ENH: Improved estimation of SegmentTubes parameters
COMP: Fixing CppCheck warnings
COMP: Remove exec flag on MergeAdjacentImage ipython notebook
Merge pull request #327 from aylward/cppcheck

Cppcheck
ENH: Add routine to save displacement fields

from computed transformations.
Merge pull request #329 from vovythevov/MiscTubeTKFixes

Fix TRE file loading in vesselview
Merge pull request #328 from ricortiz/SaveDisplacementFields

ENH: Add routine to save displacement fields
Fix ConvertTRE test baseline

Following 054b4b3de6be006b1526f71ffcdbf23ca37663ca, we need to update
ConvertTRE baseline to make sure the test compare with a correct tube file.

Change-Id: I6b01a2a8347cc517c688c123ba3f31b4ee94cd97