Add battery parameter to WH25

[StephenR0]

In this thread:

https://groups.google.com/forum/#!topic/rtl_433/nsZsA7Boggw

it was discovered that the WH25 sensor has a battery low bit. Could that be added to sdr.py? I'm using this. Thanks.

---

class FOWH25Packet(Packet):
    # 2018-10-09 19:45:12 :   Fine Offset Electronics, WH25
    # id : 21
    # temperature_C : 20.900
    # humidity : 65
    # pressure_hPa : 980.400
    # battery : OK
    # mic : CHECKSUM

    # {"time" : "2018-10-10 13:37:11", "model" : "Fine Offset Electronics, WH25", "id" : 21, "temperature_C" : 21.600, "humidity" : 66, "pressure_hPa" : 972.800, "battery" : "OK", "mic" : "CHECKSUM"}
    IDENTIFIER = "Fine Offset Electronics, WH25"

    @staticmethod
    def parse_json(obj):
        pkt = dict()
        pkt['dateTime'] = Packet.parse_time(obj.get('time'))
        pkt['usUnits'] = weewx.METRIC
        pkt['station_id'] = obj.get('id')
        pkt['temperature'] = Packet.get_float(obj, 'temperature_C')
        pkt['humidity'] = Packet.get_float(obj, 'humidity')
        pkt['pressure'] = Packet.get_float(obj, 'pressure_hPa')
        pkt['battery'] = 0 if obj.get('battery') == 'OK' else 1
        return FOWH25Packet.insert_ids(pkt)

    @staticmethod
    def insert_ids(pkt):
        station_id = pkt.pop('station_id', '0000')
        pkt = Packet.add_identifiers(pkt, station_id, FOWH25Packet.__name__)
        return pkt

[matthewwall]

patch applied at commit c34b556. thank you!