Making `cells` entry optional for compartment reports. Use default node_set is absent

The `cells` entry in [reports](https://sonata-extension.readthedocs.io/en/latest/sonata_simulation.html#reports) is optional for SONATA. If the reports `type` is `compartment`, bluecellulab requires  `cells` entry. According to SONATA [documentation](https://sonata-extension.readthedocs.io/en/latest/sonata_simulation.html#reports), for `cells`
> default is the simulation “node_set”.