Move nipype and mindboggle imports below argparse lines so that mindboggle --help can be run without environment variables set
Fix line introduced in previous commit.
Introduce LUT.py with FreeSurferColorLUT.txt numbers, names, and colors.

Rather than look up the local FreeSurferColorLUT.txt file, return values from the version used when
building mindboggle.

LUT.py contains two functions:
extract_numbers_names_colors() extracts numbers, names, and colors
representing anatomical brain regions from FreeSurfer's FreeSurferColorLUT.txt
lookup table file.

return_numbers_names_colors() returns these numbers, names, and colors.
Comment out aggregate tables in evaluate_features.py (unnecessary).
Clean up -- move xyz2nii() to utils.py, remove io_nifti.py, etc.