Add animation functions for trajectories and Voronoi cells in Jupyter notebooks

Summary

Add animate_trajectories and animate_voronoi_cells functions to PedPy that render frame-by-frame animations of pedestrian data directly in Jupyter notebooks.

Background / Context

PedPy currently supports static trajectory and Voronoi plots but has no animation capability. Animating pedestrian movement over time is a core analysis and presentation need for users working in notebook-based workflows. This is a frequently requested feature.

Technical Details

• Animations should be based on matplotlib.animation.FuncAnimation and rendered in notebooks via IPython.display.HTML
• Interface should align with existing plot_trajectories for parameter naming consistency (e.g. traj, walkable_area, color_data, color_column, cmap)
• IPython should be an optional dependency with a descriptive error when called outside a notebook environment
• New functions belong in pedpy/plotting/

Acceptance Criteria

• Usage is demonstrated in the user guide notebook
• IPython is listed as an optional dependency with a descriptive error when missing

Start in pedpy/plotting/ and read the existing plot_trajectories interface, then review the user guide notebook and the requested FuncAnimation/IPython.display.HTML approach. Done means both animation functions support the stated plotting parameters, notebook usage is documented, and missing IPython produces a descriptive error.