Add the ability to load simple text transform files.
Make DICOMCoords the default, make QForm the default.
Add NIFTI file support to the DifferenceRegistration program.
Fix metric so it uses first component of multi-component images.

The vtkImageMutualInformation metric used to crash if either of its
input images had multiple components.  This patch is the fix.  It might
also be useful to be able to select which component to use.
Fix warnings in vtkImageMutualInformation compilation.
Use only the first component when registering images.
If reader can't read file, exit instead of segfaulting.
Add help and usage to the register program.
ENH: Add writers, though they don't write yet.
Major update of the register program, includes file writing.
Add error reporting when register can't read/write transforms.
Fix variable declaration in conditional compilation.
Remove forced AIRS_USE_DICOM compile of DifferenceRegistration.
Better error message if directory is empty.
Allow setting max iterations for each resolution level.
Set the view window/level with vtkImageHistogramStatistics.
Allow the image range to be set for MI registration.
Be consistent about which image is resampled in register.

The target image was being resampled for the metric evaluation,
while the source image was being resampled for the display and
for the final output.  Now it is always the target image that is
resampled.
Use Native coords of the file type by default.
For now, only read the first component of multi-component DICOM.