Implement robust mode of mincbigaverage

[gdevenyi]

https://github.com/BIC-MNI/minc-widgets/blob/master/mincbigaverage/mincbigaverage

    --robust
        Perform robust averaging, features that are outside 1 standard
        deviation from the mean are downweighted. Works well for noisy data
        with artifacts. see the --tmpdir option if you have a large number
        of input files.

    --robust_cutoff
        Requires two floating point numbers following the argument. This
        defines the cutoff points for robust averaging. The default of
        --robust_cutoff 1.0 1.2 means that any data more than 1.0 standard
        deviation from the average will be linearly downweighted until 1.2
        standard deviations from the mean at which point the weighting will
        be zero.

https://github.com/BIC-MNI/minc-widgets/blob/master/mincbigaverage/mincbigaverage#L194-L246