niimath -n4: N4 bias-field correction

The files in this directory are a C port of Apache-2.0 licensed code:

  Insight Toolkit (ITK): itk::N4BiasFieldCorrectionImageFilter,
  itk::BSplineScatteredDataPointSetToImageFilter, itk::BSplineControlPointImageFilter,
  itk::ShrinkImageFilter.
    Copyright 1999-2019 Insight Software Consortium
    Copyright 2020-present NumFOCUS

  Advanced Normalization Tools (ANTs): N4BiasFieldCorrection command-line driver.
    Copyright (c) the ANTs contributors (ConsortiumOfANTS)

  Method: N.J. Tustison, B.B. Avants, P.A. Cook, Y. Zheng, A. Egan, P.A. Yushkevich,
  J.C. Gee. N4ITK: Improved N3 Bias Correction. IEEE Trans Med Imaging 29(6):1310-1320, 2010.

These files are distributed under the Apache License, Version 2.0 (see LICENSE in this
directory). The rest of niimath is BSD-2-Clause; build with N4=0 (make) or -DENABLE_N4=OFF
(CMake) to exclude this directory.
