Fem: Export post-processing data to VTK - fixes #5816

[marioalexis84]

It works using the Export... item in the menu bar and from Python using the writeVTK method of post-processing objects.
File extension is automatically detected from data type.

@FEA-eng

[FEA-eng]

Nice, it's working but the extension becomes *.vtk.vtu for some reason.

[marioalexis84]

the extension becomes *.vtk.vtu for some reason

Fixed.

There is a limitation in the export dialog. Currently, it's not possible to automatically select the appropriate filename extension, so the default extension is vtu. In any case, it is subsequently modified internally by the export function.
For example, if you try to save a file as result.vtu but the extension should be .vtp, this is changed internally so the file is saved as result.vtp.
The recommendation is to write the name without an extension and let the system add the appropriate one.

[FEA-eng]

Now the extension is correct, thanks.

[marioalexis84]

@chennes this PR is ready.