GNC: Update thruster model to 4th order polynomial

[DSsoto]

• Also tune gains for better performance w/ new model

[jnez71]

4th order? woah baby For future, we should move back to linearly-interpolated look-up tables.

[DSsoto]

@jnez71 If you look at the data we had from video ray with their props and shrouds, this is essentially exactly that but with better interpolation.