[guidance_v] pass only vertical thrust component to gv_adapt_run

Improve issue #527.
Get an estimate of the ctually applied thrust in the direction of the gravity vector instead of total thrust
to pass to the adaptive nominal thrust estimation.

Still should be improved by using the actual thrust after saturation/mixing...
[guidance_v] make some functions static
[guidance_v] fix vertical thrust from COMMAND_THRUST
[guidance_v] only compute thrust coeff once
[guidance_v] slightly more correct and efficient thrust_coeff calc
Merge pull request #528 from paparazzi/gv_adapt_vertical_thrust

[guidance_v] pass only vertical thrust component to gv_adapt_run

refactor and improve issue #527