fix blank page before plot while export img to pdf

[shun2wang]

Fix: https://github.com/jasp-stats/INTERNAL-jasp/issues/2782

Set onefile=FALSE : “generate a file with name containing the page number for each page”, see: https://stat.ethz.ch/R-manual/R-devel/library/grDevices/html/pdf.html ,So this needs to be discussed, I'm not sure if there are any other side effects of doing this

[vandenman]

I'm not sure if this is the correct solution, as far as I know, the issue arises as a consequence of incorrectly calling grid.newpage() somewhere in the analysis or in jaspGraphs. Also, the issue is not that there are multiple files, (which is what onefile is about) but that there is one file with multiple pages.

[shun2wang]

Seems we are not only one has this issue: https://stackoverflow.com/questions/12481267/in-r-how-to-prevent-blank-page-in-pdf-when-using-gridbase-to-embed-subplot-insi

From EJ's test case it's also to export only one img to PDF but not using export all results.