Need a chunker for cycles

As of now, argopy supports chunking along latitude/longitude/depth/time/wmo but not cycle numbers !

This was not necessary for the 'phy' dataset (core/deep data) but this is no longer the case for **BGC**

Many floats report hundreds of profiles and the erddap may return a ``Error 413 (Payload Too Large)``

See for instance [float 6901180 with +400 profiles](https://fleetmonitoring.euro-argo.eu/float/6901180):
```python
DataFetcher(ds='bgc').float(6901180)
```

This new feature should take place in https://github.com/euroargodev/argopy/blob/master/argopy/utils/chunking.py