Perform bias field correction

The following line of code removes artifacts resulting from field inhomogeneities with a bias field correction using the N4BiasFieldCorrection algorithm in Advanced Normalization Tools (ANTS) (Website download: https://github.com/stnava/ANTs.git). The input is the acpc-aligned T1 NifTi file and the output is a compressed T1-weighted NifTi file corrected for field inhomogeneities. The -d indicates the image dimensionality; -s indicates the shrink factor; -b indicates spline distance; and -c gives the convergence in number of iterations and threshold. The former three values are the standard defaults effectively used and recognized my researchers in the field.