add option to delete empty PdfPages when closed

 - PdfPages objects can be told at initialization to delete the pdf file
   when closed, if no page was actually plotted
 - keep the default to not delete empty pdf files so current behavior is
   not changed
 - also some basic tests for it
clearer comments in test
refactor docstrings of PdfPages
Merge pull request #2453 from megies/pdfpages_keep_empty

PdfPages: add option to delete empty file when closed