[pnm][util] Fix missing new line in openpnm2dgf's dgf writer
The newline was missing so everything was written in one line which leads to a wrong dgf file.
This should be backported to 3.5 after merging.
Edited by Timo Koch
The newline was missing so everything was written in one line which leads to a wrong dgf file.
This should be backported to 3.5 after merging.