FT_REGRESSCONFOUND estimates the regression weight of a set of confounds
 using a General Linear Model (GLM) and removes the estimated contribution
 from the single-trial data.

 Use as
   timelock = ft_regressconfound(cfg, timelock)
 or as
   freq     = ft_regressconfound(cfg, freq)
 or as
   source   = ft_regressconfound(cfg, source)

 where timelock, freq, or, source come from FT_TIMELOCKANALYSIS,
 FT_FREQANALYSIS, or FT_SOURCEANALYSIS respectively, with keeptrials = 'yes'

 The cfg argument is a structure that should contain
   cfg.confound    = matrix, [Ntrials X Nconfounds], may not contain NaNs

 The following configuration options are supported:
   cfg.reject      = vector, [1 X Nconfounds], listing the confounds that
                     are to be rejected (default = 'all')
   cfg.normalize   = string, 'yes' or 'no', normalization to
                     make the confounds orthogonal (default = 'yes')
   cfg.output      = 'residual' (default), 'beta', or 'model'.
                     If 'residual' is specified, the output is a data
                     structure containing the residuals after regressing
                     out the in cfg.reject listed confounds. If 'beta' or 'model'
                     is specified, the output is a data structure containing
                     the regression weights or the model, respectively.

 This method is described by Stolk et al., Online and offline tools for head
 movement compensation in MEG (Neuroimage, 2013)

 To facilitate data-handling and distributed computing you can use
   cfg.inputfile   =  ...
   cfg.outputfile  =  ...
 If you specify one of these (or both) the input data will be read from a *.mat
 file on disk and/or the output data will be written to a *.mat file. These mat
 files should contain only a single variable, corresponding with the
 input/output structure.

 See also FT_REJECTCOMPONENT, FT_REJECTARTIFACT