feat: Ignore data and image files within the gnuplot dir
feat: Ignore data and image files within the gnuplot dir

Added all gnuplot/*.dat and gnuplot/*.png to be ignored, since these are
user generated files.

Signed-off-by: Sebastian Niemann <c15bc648238b8fcefd2112b1efddcb822e686c46@sra.uni-hannover.de>

Mantella/Mantella,SebastianNiemann/Mantella,SebastianNiemann/Mantella,Mantella/Mantella,SebastianNiemann/Mantella,Mantella/Mantella