Add LN2_SENSITIVITY and LN2_SPECIFICITY programs

[ofgulban]

As proposed by https://doi.org/10.1093/cercor/bhad151 , it might be handy for users to be able to compute voxel wise sensitity and specificity maps. We can take 4D nifti files as input, for instance each volume having one type statistic (e.g. beta values) per condition.