Fixed sensor.accelerometer gravity sign for consider_gravity = True case

[vapsik]

Issue

In the case of consider_gravity = True the current sensor.accelerometer sensor accounts for gravity vector incorrectly — $$a_{ \text{inertial} } = a_{\text{ENU}} + g$$ but since the gravity vector has components $g = (0,0,-9.8)$ in ENU frame then it must be $$a_{ \text{inertial} } = a_{\text{ENU}} - g$$.

A simple example showcasing the current implementation being incorrect is with a stationary state (1) and airless free-falling rocket (2) - both with ENU-aligned accelerometers: 1) during stationary phase where $a_{\text{ENU}} = 0$ the consider_gravity = True accelerometer reading shows $a_{ \text{inertial} } = (0,0,-9.8)$ where as real IMU would show $a_{ \text{inertial} } = (0,0,+9.8)$ as normal force upward from the ground must negate the effect of gravity for achieving the stationary state. 2) as a rocket goes into (airless) free fall real IMU would show inertial acceleration of 0 (weightless) but with current implementation in free fall or near apogee before parachute opening the consider_gravity = True accelerometer shows total acceleration of nearly 19.6 showcasing the wrong gravity sign usage.

Changes

In accelerometer.py changed inertial_acceleration = Vector(u_dot[3:6]) + gravity into inertial_acceleration = Vector(u_dot[3:6]) - gravity.

Test file showcasing the bugs (made with Claude)

repro_gravity_bug.py

[Gui-FernandesBR]

@vapsik tests not passing on CI... Could you take a look pleasE?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (7e785a6) to head (012fd5d).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1175      +/-   ##
===========================================
+ Coverage    89.96%   91.13%   +1.16%     
===========================================
  Files          131      131              
  Lines        17527    17560      +33     
===========================================
+ Hits         15769    16003     +234     
+ Misses        1758     1557     -201     

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