STYLE: Remove unused code from vtkSeedTracts

Extent from the input tensor field was not used.
BUG: Re-organize tractography seeding pipeline to work with VTK6. See #3845

This commit fixes pipeline incorrectly setup in r23121 by using output
port instead pipeline information (InputROIPipelineInfo)
STYLE: Remove commented code from Tractography seeding modules
ENH: Do not set workaround to associate scalar with input tensor field.

This is not required when using VTK6.
ENH: Simplify Diffusion tensor reader using FindPoint with both VTK 5 and 6
BUG: Fix return value in diffusion tensor reader "RequestData()"
BUG: Update diffusion tensor reader to force origin/spacing. Fixes #3845

This commit fixes a regression introduced in r23122 and forces the
origin/spacing for VTK 6 similarly to what was done for VTK 5.