Initial support for Beurer BF500

[ghtyrant]

Beurer's BF500 seems to be a mix of BF105 and BF600. Characteristics determined by using BLE Scanner and trial & error. I did not verify that CHARACTERISTIC_BEURER_BF500_SCALE_SETTING and CHARACTERISTIC_BEURER_BF500_REFER_WEIGHT_BF are correct or do what they are supposed to do as they were not implemented by other scales.

What works:

• Transferring a user from the app to the scale (e.g. deleting all users on the scale before connecting with the app)
• Taking measurements (Weight, BMI, Body water, Body fat)
  • Body Water required the same calculations as with BF105 before working correctly, the rest was correct out-of-the-box

What doesn't work:

• I've set up a user with my correct birth year (1988), but on the scale the user was created with birth year 1995 - day and month were correct though. I don't know if this is a bug in the app or something went wrong when sending the data via Bluetooth.
  When reading the user list from the scale, the birth year is transferred and parsed correctly, so updating the year on the scale worked.
• Automatic user detection doesn't seem to work. Someone else got on the scale and the scale still assigned the measurement to my user (even with vastly different weights). I don't know if this is an issue with the app (Smart User assignment is active) or the scale.

Let me know if you need any other data/characteristics from the scale to improve this first draft.