Move label interpolation into its own class.

This causes a lot of duplication between the label interpolator and
the Gaussian interpolator, but eventually a lot of the shared code can
go into a common base class (along with a lot of code from the b-spline
interpolator and the sinc interpolator).
ENH: Add a program that computes Green's strain from a transform.
Add a WarpImage program.
Fix coordinate system for WarpImage and ComputeStrain.

ANTs internally uses DICOM patient coords for everything, even though
NIFTI files do not.  Need to change the coords for the transforms in
order to make them work on the NIFTI files in NIFTI coords.
Add a program to display strain glyphs and lots of other changes.
ENH: Use arrow glyphs to display strain.
Use the header matrix for images (but not for warp grid, yet).
Add an LSM reader for miscrosope images, no lzw.
Remove missing class from the CMakeLists.txt.
Use B-spline to get better derivatives for strain.
Add an lsm-to-nifti converter.
Make lsmtonifti converter efficient for unset options.
Add an option to read tiff as well as lsm.

This will often not work, because the VTK TIFF reader in VTK 5.x
cannot read stacks, and because if it detects a color table then
it will produce an RGB image, which is no good for this program.
Add principal component calculation.
Add more options to the ComputeStrain program.
Add instructions to use "-h" to the lsmtonifti executable.