Attitude model

[Mr-Medina]

Feature

this issue cover point 2 and part of 3 in #190

Desired Behavior / Functionality

Adding a spacecraft attitude model specifying initial attitude conditions and attitude evolution through time (with external attitude disturbances).

What Needs to Be Done

Create an attitude model class with initial conditions.
Specify body fixed pointing vector (used for plotting, testing and orientation).
Add "update attitude" function, updating spacecraft state (roll, pitch, yaw angles, pointing vector orientation, angular velocity and acceleration).

How Can It Be Tested

Check pointing vector evolution with zero initial angular velocity, angular velocities around specific axes testing time between full rotations. Do the same with (constant) angular acceleration.