feat: more customization APIs

[moe-ad]

See #465 for context.

[AlejandroFernandezLuces]

Thanks for moving the changes here!

On the add_point_labels, after looking into it, the main difference between adding a label and adding text, is the fact that the label will always be facing the user, while adding text will rotate with the scene. This is a reason good enough to have a public API for this. Although, the reason why PyVista calls it point_label is because in VTK you can associate labels to their different types of objects: points, edges, vertex, etc. So, I would rename it to simply add_label, to be as generic as possible. But agreed that this is a legitimate use case for the public API.

For the clear method, I still think it kind of leads to confusion for users. I think it leads to think that the flow would be something like this:

plotter.plot(mesh)
plotter.show()
plotter.clear()
plotter.plot(mesh)

which breaks for PyVista, our major backend. We could make a workaround in the PyVista backend by recreating the plotter instance after a show(), so externally you don't need to create another instance, but I feel this is incredibly hacky.

I understand your interactive argument, but it compromises scripting and coding in my opinion. What do yout think? Do you see any way of clarifying this for the users elegantly?