Add voronoi diagram from optimiser queries

[BradyPlanden]

Feature description

Add a plotting method that generates a voronoi diagram from the locations visited during optimisation. This can be an optional arg passed to pybop.plot2d. This method will only use the previous optimiser visits without generating new ones. Further information on voronoi diagram, as well as Pints' implementation using Matplotlib.

Motivation

This provides a fast implementation to view the cost landscape for convergence diagnostics. Users can then determine if they wish to investigate further with the currently reconstruction implementation with pybop.plot2d

Possible implementation

• Add a plotting method that generates the voronoi diagram (SciPy has functionality for this).
• Extract voronoi vertices from the above, and pass them to a Plotly surface object

Additional context

No response