docs: def convert(self,label,units=None,conversion_function=convert_time):
          """ converts a dimension in place """
          label_no = self.get_label_no(label)
          new_label, new_column = self.get_converted(label_no,units,conversion_function)
          labels = [LabelDimension(l) for l in self.labels]
          labels[label_no] = new_label
          matrix = self.matrix.copy()
          matrix[:,label_no] = new_column
          return LabeledMatrix(matrix,labels)