Gravity disturbances in attitude model

# Feature

Adding the gravity disturbance torque to the attitude model. Part 3.3 of #190

## Desired Behavior / Functionality
The modeled spacecraft should have a torque acting on it due to the gravity of the Earth. The model is taken from https://doi.org/10.1016/j.asr.2018.06.025, and will include up to and including the J2 effects. According to the paper, effects from J2 onward are many orders of magnitude smaller.

## What Needs to Be Done
A location is already determined in the attitude model where this implementation should go. Hence, the gravity model needs to be implemented as a function, and then the correct actor/simulation parameters should be implemented.

## How Can It Be Tested
By creating a pytest test. The strength of the gravity field in a particular orbit should be known, and this can be combined with the mass of the satellite. Hence the implementation of the gravity disturbance torque, as well as checking whether it is using the geometric model correctly can be checked. Whether the correct attitude is achieved after getting the torque is tested within the attitude model.