FT_VOLUMEWRITE exports anatomical or functional volume data to a Analyze
 or BrainVoyager file. The data in the resulting file(s) can be
 further analyzed and/or visualized in MRIcro, SPM, BrainVoyager,
 AFNI or similar packages.

 Use as
   ft_volumewrite(cfg, volume)
 where the input volume structure should represent an anatomical MRI
 that was for example obtained from FT_READ_MRI, the source
 reconstruction results from FT_SOURCEANALYSIS, the statistical
 results from FT_SOURCESTATISTICS or an otherwise processed anatomical
 or functional volume.

 The configuration structure should contain the following elements
   cfg.parameter     = string, describing the functional data to be processed,
                         e.g. 'pow', 'coh', 'nai' or 'anatomy'
   cfg.filename      = filename without the extension
   cfg.filetype      = 'analyze', 'nifti', 'nifti_img', 'analyze_spm', 'mgz',
                         'vmp' or 'vmr'
   cfg.vmpversion    = 1 or 2 (default) version of the vmp-format to use

 The default filetype is 'nifti', which means that a single *.nii file
 will be written using the SPM8 toolbox. The 'nifti_img' filetype uses SPM8 for
 a dual file (*.img/*.hdr) nifti-format file.
 The analyze, analyze_spm, nifti, nifti_img and mgz filetypes support a homogeneous
 transformation matrix, the other filetypes do not support a homogeneous transformation
 matrix and hence will be written in their native coordinate system.

 You can specify the datatype for the analyze_spm and analyze formats using
   cfg.datatype      = 'bit1', 'uint8', 'int16', 'int32', 'float' or 'double'

 By default, integer datatypes will be scaled to the maximum value of the
 physical or statistical parameter, floating point datatypes will not be
 scaled. This can be modified with
   cfg.scaling       = 'yes' or 'no'

 Optional configuration items are
   cfg.downsample    = integer number (default = 1, i.e. no downsampling)
   cfg.fiducial.nas  = [x y z] position of nasion
   cfg.fiducial.lpa  = [x y z] position of LPA
   cfg.fiducial.rpa  = [x y z] position of RPA
   cfg.markfiducial  = 'yes' or 'no', mark the fiducials
   cfg.markorigin    = 'yes' or 'no', mark the origin
   cfg.markcorner    = 'yes' or 'no', mark the first corner of the volume

 To facilitate data-handling and distributed computing you can use
   cfg.inputfile   =  ...
 If you specify this option the input data will be read from a *.mat
 file on disk. This mat files should contain only a single variable named 'data',
 corresponding to the input structure.

 See also FT_SOURCEANALYSIS, FT_SOURCESTATISTICS, FT_SOURCEINTERPOLATE