Update TubeTK repository location to GitHub

The TubeTK repository has moved to GitHub.
Merge pull request #1 from jamiesnape/UpdateTubeTKRepositoryLocation

Update TubeTK repository location to GitHub
Merge pull request #2 from TubeTK/workflow

Workflow
ENH: Uses improved superbuild structure from Slicer.

Co-authored-by: Jean-Christophe Fillion-Robin<jchris.fillionr@kitware.com>
Merge pull request #3 from chrismullins/improve-superbuild-with-workflow

ENH: Uses improved superbuild structure from Slicer.
STYLE: Fixes minor spelling and python errors
Merge pull request #4 from chrismullins/workflow-fixes

STYLE: Fixes minor spelling and python errors