Depending on input data type, the storage type is either
        "field" (complex) or "phase" (real).

def storage_type(self):
        """Depending on input data type, the storage type is either
        "field" (complex) or "phase" (real)."""
        nf = np.load(str(self.path), mmap_mode="c", allow_pickle=False)
        if np.iscomplexobj(nf):
            st = "field"
        else:
            st = "phase"
        return st