TEST: write some tests for dynamics

[AlexS12]

src/dynamics/*.jl contains the dynamic systems.

Some tests must be written for this part. However, it is difficult to find independent validation values.

Another way of writing some of these tests would be to test one model against some others in equivalent conditions. For examples, six_dof_euler_fixed_mass is supposed to provide similar results as six_dof_quaternion_fixed_mass if the attitude is not extreme (theta near 90º)

Some projects test the full simulator response: once an aircraft is implemented they integrate a solution for the aircraft response to a given input. Even if this this is not a unit test, this might be useful at some point.