Update the client to deal with stream from the datatransfer service

[JulienPeloton]

PoC:

fink_datatransfer.py -h
usage: fink_datatransfer.py [-h] [-topic TOPIC] [-limit LIMIT] [-outdir OUTDIR] [-partitionby PARTITIONBY] [-batchsize BATCHSIZE] [--restart_from_beginning]
                            [--verbose]

Kafka consumer to listen and archive Fink streams from the data transfer service

optional arguments:
  -h, --help            show this help message and exit
  -topic TOPIC          Topic name for the stream that contains the data.
  -limit LIMIT          If specified, download only `limit` alerts from the stream. Default is None, that is download all alerts.
  -outdir OUTDIR        Folder to store incoming alerts. It will be created if it does not exist.
  -partitionby PARTITIONBY
                        Partition data by `time` (year=YYYY/month=MM/day=DD), or `finkclass` (finkclass=CLASS), or `tnsclass` (tnsclass=CLASS). Default is
                        time.
  -batchsize BATCHSIZE  Maximum number of alert within the `maxtimeout` (see conf). Default is 1000 alerts.
  --restart_from_beginning
                        If specified, restart downloading from the 1st alert in the stream. Default is False.
  --verbose             If specified, print on screen information about the consuming.

Note that unlike the livestream service, the schema for these alerts is stored in a separate topic (topic_name_schema) that is polled simultaneously when retrieving alerts.

To do:

• new script
• documentation in the README
• documentation in the readthedocs (along with the datatransfer service).