Fix ROT decode rounding

[davidalo]

Hello,

When encoding and decoding some messages I found an inconsistency in ROT (turn). For some values it's inconsistent because to_turn function is truncating instead of rounding.

You can see the effect in the following screenshots:

I added a new test test_rot_encode_decode to cover this.

Related PR: https://github.com/M0r13n/pyais/pull/74 . I see that to_turn was rounding in the past, but it was changed for some reason. Could you shed some light on this?

[M0r13n]

Hey @davidalo,

The GPSD project - the de facto reference documentation for this project - does not round: https://gitlab.com/gpsd/gpsd/-/blob/master/devtools/ais.py#L117. Therefore, I dropped the round statement, because it caused some trouble for other users.

However, I am not 100% sure, since the values for ROT vary slightly from decoder to decoder. Depending on the decoder, there seems to be a certain confidence interval of +/- 5 degrees, in which all available decoders scatter. 🤷

For example the very same messages are decoded differently:

• !AIVDO,1,1,,A,10000Nh200000000000000000000,0*01
  • Aggsoft: 4
  • Maritec: +2.9
  • pyais: 2.0
• !AIVDM,1,1,,A,14QIG<5620KF@Gl:L9DI4o8N0P00,0*28:
  • Aggsoft: 25
  • Maritec: +25.7
  • pyais: 25

I am inclined to accept your PR with the addition to round to two decimals places.