matrixShuffleZscorePerSequence: Obtain z-score for motif column shuffling
In PWMEnrich: PWM enrichment analysis

All PWMs are shuffled at the same time. This function would be too slow to produce empirical P-values, thus we return a z-score from a small number of shuffles.

1	matrixShuffleZscorePerSequence(scores, sequences, pwms, cutoff = NULL, B = 30)

`scores`	a set of already calculated scores
`sequences`	either one sequence or a list/set of sequences (objects of type DNAString or DNAStringSet)
`pwms`	a list of PWMs
`cutoff`	if NULL, will use affinity, otherwise will use number of hits over this log2 odds cutoff
`B`	number of replicates, i.e. PWM column shuffles