Fix #26
Fix #26

Looking for external data in known subfolders from root directory. This change
allows to have a build directory inside the source tree (with the superbuild
option).

InsightSoftwareConsortium/ITKExamples,InsightSoftwareConsortium/ITKExamples,InsightSoftwareConsortium/ITKExamples,InsightSoftwareConsortium/ITKExamples,InsightSoftwareConsortium/ITKExamples