Creating an geometric model

# Feature
This issue covers point 1 of issue #190.

## Desired Behavior / Functionality

Adding a geometric model to the model. This is done with two options; option 1 is to model a basic cuboid with width, length, and height. Option 2 is to import an .obj file, and to use the mesh from that.
The outputs of the geometric model are currently planned to be the moments of inertia, and the center of gravity.

## What Needs to Be Done

Several functions/classes will be created. Firstly, in the file actor_builder.py there will be a function set_cubiod_geometric_model to add then cuboid geometric model to the actor. There will also be a similar function, set_geometric_model_from_import for the option to import a mesh from the .obj file.
Then a new file is implemented, geometric_model.py, which contains two classes, CuboidGeomtericModel, and ImportGeometricModel. These two classes will each have the function to find the moment of inertia, and the centre of gravity. Additionally, the mass of the satellite is stored in the class.
Depending on what is needed for the disturbances that will be added, more parameters may need to be added to these two classes.

## How Can It Be Tested
Lastly, two tests will be added in the file actor_builder_test.py. One to test the cuboid geometric model, and one to test the import. The import test contains the ability to view the imported model in the correct coordinate system.