Replace conversion tool with ccx2paraview

[fsimonis]

The frdConverter has a couple of downsides:

• it is foreign code in our repository
• it requires mono, which is pretty dependency-heavy (~700Mb on Ubuntu)
• it is fragile to use as it needs an absolute path
• there is no way to easily install it user/system-wide as it would require some wrapper script
• it doesn't produce a .series / .pdv file which links output to time.

My suggestion is to replace it with ccx2paraview as it is easy to install, use, and produces .pdv files.

I am currently trying to get it to a state where installing it is as easy as pipx install ccx2paraview. https://github.com/calculix/ccx2paraview/pull/35

This would simplify the CalculiX repo, its documentation, post-processing in the tutorials, and the VM.