Attitude model

# 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.