Streamline Orientations api

[stefsmeets]

Some ideas to streamline the api:

• [x] cartesian_to_spherical -> Orientations.get_vectors_spherical()
• [x] plots.rectilinear_plot(orientations=rotational_traj) -> Orientations.plot_rectilinear()
• [x] plots.bond_length_distribution(orientations=rotational_traj, bins=1000) -> Orientations.plot_bond_length_distribution()
• [x] plots.unit_vector_autocorrelation(orientations=rotational_traj) -> Orientations.plot_unit_vector_autocorrelation()
• [x] autocorrelation = mean_squared_angular_displacement(traj.vectors) -> Orientations.mean_squared_angular_displacement()
• [x] rotations.py -> orientations.py?

Also take note of #301

[stefsmeets]

Also consider changing the initializer:

Orientations.from_trajectory(...)

Returns:

@dataclass
class Orientations:
    vectors: np.ndarray