Function returns the minRQ, defined in Andersen et al. (2012).
Assume there is \(N\) equispaced returns in period \(t\). Let \(r_{t,i}\) be a return (with \(i=1, \ldots,N\)) in period \(t\).
Then, the minRQ is given by
$$
\mbox{minRQ}_{t}=\frac{\pi N}{3 \pi - 8} \left(\frac{N}{N-1}\right) \sum_{i=1}^{N-1} \mbox{min}(|r_{t,i}| ,|r_{t,i+1}|)^4
$$