groupdiff_tau()
computes \(min(x/y, y/x)\), i.e. the smallest symmetric ratio between \(x\) and \(y\)
that is smaller than 1. If \(x\) is a vector, the symmetric ratio between all
elements in \(x\) is computed.
groupdiff_absdiff()
computes \(max(abs(x-y, y-x))\), i.e. the smallest absolute difference
between \(x\) and \(y\).
If \(x\) is a vector, the symmetric absolute difference between all elements in \(x\) is computed.