zipper should not change source file state

[psychemedia]

At the moment, if you request zipped files with cleared cells, the original notebooks are processed to clear their cells, and then the cleared output files are zipped. An option should be available to leave the original files in their original stage, so that eg run notebooks used for testing can be left as is, whilst their cleared cell output variant is add to the zip file.

[psychemedia]

Fixed - files for execution for zipping are now copied to a tmpfile in the same dir, executed, then written to zipfile with original name.