add further options in plot_voronoi_cells

[JuleAdrian]

Add further optional arguments for plot_voronoi_cells, such as the line width of the boundaries of the voronoi cells and the size of the dots indicating the current ped positions.
Left picture shows the default settings and the right picture custom settings that could be achieved.

[mirakuepper]

The size of the dots is already implemented. At least I could change the size of the dots with the optional argument "ped_size= ...".
However, this is not listed in the API (or I am blind and did not see it).

[mirakuepper]

Especially the label of the colorbar should be adjustable. Currently, if you do not use density or speed as color for the cells, the only way to get a correct label is to hide the colorbar and build a new one.