BUG: Fix gravity term in accelerometer (#29)

* Fix gravity term in accelerometer

* Fix accelerometer gravity model in tests

Author: zuorenchen

rocketpy/sensors/accelerometer.py

@@ -232,7 +232,7 @@ def measure(self, time, **kwargs):
         gravity = (
             Vector([0, 0, -gravity]) if self.consider_gravity else Vector([0, 0, 0])
         )
-        inertial_acceleration = Vector(u_dot[3:6]) + gravity
+        inertial_acceleration = Vector(u_dot[3:6]) - gravity
 
         # Vector from rocket cdm to sensor in rocket frame
         r = relative_position

tests/unit/sensors/test_sensor.py

@@ -272,7 +272,7 @@ def test_noisy_rotated_accelerometer(noisy_rotated_accelerometer, example_plain_
 
     # calculate acceleration at sensor position in inertial frame
     relative_position = Vector([0.4, 0.4, 1])
-    inertial_acceleration = Vector(U_DOT[3:6]) + Vector([0, 0, -GRAVITY])
+    inertial_acceleration = Vector(U_DOT[3:6]) - Vector([0, 0, -GRAVITY])
     omega = Vector(U[10:13])
     omega_dot = Vector(U_DOT[10:13])
     acceleration = (