added a method to compute whitening matrix
added a method to compute whitening matrix

drsternburg/bbci_public,IrinaNicolae/bbci_public,bbci/bbci_public,IrinaNicolae/bbci_public,drsternburg/bbci_public,IrinaNicolae/bbci_public,svendaehne/bbci_public,bbci/bbci_public,svendaehne/bbci_public,svendaehne/bbci_public,drsternburg/bbci_public,bbci/bbci_public