dev: upgrade to roxygen 4.0.2
dev: tell rstudio to use devtools
First version of code to fix blurry pptx pdfs

* motivated by the fact that PowerPoint 2010 on windows and PowerPoint 2011 on Mac
  replace pdfs with pngs resmapled to 72 dpi, which can look very ugly
first test - is.pptx - and testdata
tets unzip_pptx
test for equality of re-zipped pptx

* failing due to included dirs
* add utility function for testing zip equality
Fix rezip pptx equality don't include dirs
test that we can cope when files has length 0
test when specifying files to freshen
check for error when file exists & action='error'
test convert_pptx_pdfs on new no_pdf file

* fails due to different behaviour when no action required
more consistent behaviour when no action required
check conversion of test_pdf.pptx

* just one image different
prefix nat.utils
Document and export pptx functions
add pngres function
Only regenerate low resolution pngs

* will be skipped if they already meet the resolution requirement
Fix check warning about long example line
all.equal.zip has new strict argument

* strict checks for exact equality of contents (by order, date etc)
check if convert_pptx_pdfs only touches file if reqd

* unfortunately it doesn't yet