Correct parsing of RMU data fields

[elupus]

• Setting RMU target temperature needs 16 bit signed values
• Setpoint for offsets with RMU does not have any scaling applied
• Correct jump of outdoor temperature when received from RMU since pump seem to apply the scaling in reverse

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (3eb7b88) 57.46% compared to head (c109a85) 57.72%.

Files	Patch %	Lines
nibe/connection/nibegw.py	76.47%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #145 +/- ## ========================================== + Coverage 57.46% 57.72% +0.26% ========================================== Files 14 14 Lines 1213 1230 +17 ========================================== + Hits 697 710 +13 - Misses 516 520 +4 ```

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[yozik04]

Do you want to cover encode as well with a test?

[elupus]

Yes probably. Will look at it. I think some back to back roundtrip testing of decode/encode would be enough there.