modified HM-WDS30-OT2-DS18B20 with better defaults and up to 4 sensors

[harvey637]

Slightly modified:

• defaults (unused temperatures) now read as -150°, not as 0° (which could be a true temperature)
• 1, 2, 3, 4 sensors possible
• 1 sensor only gives own temperature, no differences, unused temps are -150°
• 2 sensors (like original) give two temperatures, and difference 2-1 and difference 1-2
• 3 sensors give two temperatures (1, 2), and difference 2-1 and third temperature
• 4 sensors give all four temperatures, no differences

[jp112sdl]

That's nice. Thanks!