Relying on data_returned

[eimrek]

The data_returned field in the /structures endpoint of the OPTIMADE specification is a 'SHOULD' (meaning, it's recommended, but not a requirement). For large databases (such as the li-ion-conductors one here: https://optimade.materialscloud.org/archive/vg-ya/v1/structures), we time out the count and return null there. The optimadeclient, however, seems to rely on this. This should be fixed, if possible.