Subissue: Enable vtk ASCII output for ghost data

[holke]

ASCII sub-issue of #1487

Feature request

Currently if VTK output is called with the "write_ghosts" flag, then still "0" is written for any user data.
Instead, if "write_ghosts" is set and user data is provided, then the values at the ghost element indices should be written our correctly.

Is your feature request related to a problem? Please describe.
It is not properly documented that we do not write the ghost data which leads to misunderstanding of the function and wrong assumptions on the actual data present.

Describe the solution or feature you'd like
If "write_ghosts" is set in t8_forest_write_vtk_ext then any provided data must contain entries for ghost values and these should be written into the vtu file in ASCII mode.

Describe alternatives you've considered
Alternatively at leas a "nan" or "inf" value should be written instead of "0" in order to not confuse it with real data.

Estimated priority
Which of these is most applicable (remove the others):

"Priority: low" Should be solved eventually

Additional context
This came up during a debugging session with an applications and the users did not know that we do not write ghost values and were confused about 0 being written. They thought their code was not setting the ghost values correctly.