ENH: rename vtkNRRDReader,vtkNRRDWriter -> vtkTeemNRRD{R,W}

Discussed on hangout this week. vtkNRRDReader is ambiguous with vtkNrrdReader, both to a casual reader and to the build system on case-insensitive filesystems.

There are five extensions using these classes:

• Sequences: https://github.com/SlicerRt/Sequences/pull/47
• MultiVolumeExplorer: https://github.com/fedorov/MultiVolumeExplorer/pull/40
• SlicerDMRI: https://github.com/SlicerDMRI/SlicerDMRI/pull/103
• UKFTractography: https://github.com/pnlbwh/ukftractography/pull/100
• SlicerCIP: https://github.com/acil-bwh/SlicerCIP/pull/23

I will make PRs to those projects and link them here. Need to coordinate merges.