Add a nifti-to-dicom converter.

This will take a NIFTI file as input, and optionally a series of
DICOM files as well, and will then produce a series of DICOM files
using the pixel data in the NIFTI file.
Implement the series-description, series-number options.
Add some well-known frames of reference.
Add scaling to keep values within a resonable short range.
Add niftitodicom option to choose ax, sag, or cor output.

This should complete the niftitodicom tool.  One thing that I might
add in the future is some code to set the in-plane phase encoding
direction, if the NIFTI file provides that information.