improvement for sensor_type

[dijkstra1930]

For csv

sensor_types
temp
digital_temp
light
pressure
humidity
motion
audio_p2p
acc_x
acc_y
acc_z

• first line correctly has field name, and list of "old" sensor names (missing RSSI, etc.)
• so we need to do something with device name, or some update device table entry

For json

{"device_type":"firefly_v3", 
  "sensor_type":"temp,digital_temp,light,pressure,humidity,motion,audio_p2p,acc_x,acc_y,acc_z"}

• adds "device_type, not present in csv (?)
• jams sensor together a single string
• calls it "sensor_type", not "sensor_types"