Add vector field support

[phohenberger]

This adds vector field support by:

• Adding a new arrow mesh with the scale parameter
• VectorField class that is analogue to Particle but contains different parameter direction for the direction of the vectors. The current design is such that each position is assigned the corresponding direction vector in the list of given positions and vectors.
• optional vector_field parameter for the visualizer that works the same as particles, but is declared seperately and takes in a VectorField instance

Note that the initial rendering is quite slow for large numbers of vectors. This could be optimized somehow.

Example Syntax:

v_mat = vis.Material(colour=np.array([0, 0, 255]) / 255)
v_mesh = vis.Arrow(scale = 10, material = v_mat)
v_field = vis.VectorField(name="flow", mesh=v_mesh, position=pos, direction=dir)

visualizer = vis.Visualizer(particles=[...], vector_field=[v_field], frame_rate=30)

The variables pos and dir are of the shape (n_steps, n_vectors, n_dims = 3)

[SamTov]

Looks great. There were some small things like type hints, ensuring all modules have their full import and new lines at the end of the page. But as they were all very small I just did it myself.

Thanks for the awesome PR! @christophlohrmann This will solve some of your visualisation requests.