def _get_const_period(self, src_cls, dest_cls):
        if dest_cls == ('global', 'const'):
            return None
        elif src_cls == ('global', 'slices'):
            return self.get_multiplicity(src_cls) / self.get_multiplicity(dest_cls)
        elif src_cls == ('vector', 'slices'): #implies dest_cls == ('time', 'samples'):
            return  self.n_slices
        elif src_cls == ('time', 'samples'): #implies dest_cls == ('vector', 'samples')
            return self.shape[3]
        assert False #Should take one of the above branches
Get the period over which we test for const-ness with for the
dataset/ETHPy150Open moloney/dcmstack/src/dcmstack/dcmmeta.py/DcmMetaExtension._get_const_period