Correct inconsistence of convention system

[tsaoyu]

There are some inconsistence between convention system and our code. We need update them on WiKi page.

[tsaoyu]

In more detail we need document the following:

• Zero position and its reference
• Direction convention of signs
• Unit type

We can use hardware to check the consistency of conventions with real hardware. Servo test script is reusable to test sail angle/ rudder angle.

[smaria]

I quite liked the proposal at the southampton python user group, of having classes that return angles etc. in different reference systems.

A lot of conventions are already documented on the naming conventions page, but may need updating...

Which conventions are we missing from that page?