Add station identifier field to chasemapper json

[lebaston100]

Feel free to deny since it technically breaks "spec". radiosonde_auto_rx also sends that field (https://github.com/projecthorus/radiosonde_auto_rx/blob/d3b2d06a098406c6232799e385752693d0c3f0d8/auto_rx/autorx/ozimux.py#L157). I use it to differentiate between packets coming from different receivers for data logging purposes. Doesn't seem to be used in chasemapper (yet).