Simple integrator for orientation quaternions. Given an orientation quaternion, initial angular velocity, an inertial tensor and a function that gives torque as a function of orientation, this package allows you to compute the time evolution of the orientation.

Currently relatively untested (despite the code coverage) and I have no idea of the accuracy of the algorithm.

```
using QuaternionIntegrator, Quaternions, LinearAlgebra
# Constant torque around y axis
torque(q) = [0.0, 1.0, 0.0]
# Simplest diagonal inertial tensor
I = diagm([1.0, 1.0, 1.0])
# Starting orientation
q0 = orientation([0.0, 1.0, 0.0], π/2)
# Starting angular velocity (not rotating)
ω0 = [0.0, 0.0, 0.0]
# Integration time step
∆t = 0.01
# Get orientation and angular velocity after one timestep
q1, ω1 = integrate(q0, ω0, I, ∆t, torque)
# Compute 1000 time steps ahead
qn, ωn = integrate(q0, ω0, I, ∆t, torque, 1000)
```