add Permutation test for weighted Score

[Peter]

Add this function in base class taking target, value, weight, and number of permutations, and perhaps a treshold telling us that we are only interested in p-values below this limit. The perumtation run shall of course be exit when we can be sure that the final p-value will not be below the treshold, e.g. quit when (O-E)^{2/E goes above some limit (depending on p-treshold).}