Returns the keys of the GridSpace

        Args:
            full_grid (bool, optional): Return full cross-product of keys

        Returns:
            List of keys

def keys(self, full_grid=False):
        """Returns the keys of the GridSpace

        Args:
            full_grid (bool, optional): Return full cross-product of keys

        Returns:
            List of keys
        """
        keys = super(GridSpace, self).keys()
        if self.ndims == 1 or not full_grid:
            return keys
        dim1_keys = sorted(set(k[0] for k in keys))
        dim2_keys = sorted(set(k[1] for k in keys))
        return [(d1, d2) for d1 in dim1_keys for d2 in dim2_keys]