VisualPIC 0.5 (preview 2)

A couple of productive coding weeks thanks to the current coronavirus "retreat" make it worth publishing another release with some brand-new features:

1. First things first: there's a new "main" image for VisualPIC!
   This new render (entirely made in VisualPIC with no additional editing), apart from looking nice, showcases that now you can not only display fields in 3D but also...

2. ... particles! Yes, now the ParticleSpecies can also be shown along volumetric fields. This has been implemented using the efficient vtkOpenGLSphereMapper, which allows displaying millions of particles as spheres with a high framerate even in a conventional laptop. Each of these particles can have individual size and color, allowing for a high tunability of the appearance of each species. The visual properties of the displayed species can be interactively modified in the GUI as well as in the code.

3. There is now a new example which showcases the basics of particle rendering.

4. The size of the 3D render can now be scaled along any direction (thanks @delaossa for the suggestion!). This can be seen in the new example, where the particle beam is stretched along z.

5. pyvista has now been added as a requirement. This simplifies the implementation of vtk components, but whether it is truly needed will be reevaluated before finally releasing v0.5.

6. The implementation of the internal normalization applied by VisualPIC to scale the dimensions of 3D fields and particles from the original units to vtk units has been changed.

7. As always, several bug fixes and changes here and there.