Doc: openPMD to GDF and VTK

[ax3l]

Add links to VTK and GDF converters.

Adds @hightower8083's openPMD to VTK converter project: #1 (comment) .
Adds @KseniaBastrakova's openPMD to GDF to openPMD converter project.

Thank you both for the great converters! 🚀 ✨

[hightower8083]

you welcome Axel -- glad to be of a help!
One more thing -- presently my VTK converter is based on pyvtk, which is limited to legacy VTK format, and is terribly slow. At some point i'll need to add tvtk version, which is fast but is a much heavier dependancy (mayavi). Cheers!

[ax3l]

sounds great!
If you want, you can also base the input reader on openPMD-api instead of openPMD-viewer. This would make it more versatile, e.g. supports ADIOS as well.

s/Alex/Axel/g ;-)

[hightower8083]

D’oh ! damn these chubby fingers 😁