converts a dimension in place

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)