do not allow negative channel numbers in MCA rois
(reported by @claustre)
currently mca's accept negative channel numbers in rois.
This should raise an error:
mymca._rois.set('roi1', -1000,1000)
(reported by @claustre)
currently mca's accept negative channel numbers in rois.
This should raise an error:
mymca._rois.set('roi1', -1000,1000)