PGN 130306 True Wind Angle should be +/-pi.

[goergevo]

environment.wind.angleTrueWater should be generated from the incoming 0 - 2pi range by converting it to +/-pi. It should be handled exactly like environment.wind.angleAppearent.
On my Boat, I had the problem that TWA generated by my wind instruments received via n2k were 0..360 degree and not consistent with the value that the derived data plugin calculates. With the above described conversion, the received values and the ones calculated locally by the derived data plugin are identical.
It is now also consistent with the signalk to n2k conversion. In that code, TWA is converted from +/-pi into the 0-2pi range before sending it out.