Pdf images resizing (rebased onto develop)



This is the same as gh-500 but rebased onto develop.

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When reviewing #498 @mtbc pointed out that while my rescaling looked fine for the html pages, some of the screenshots were too big in the pdf, so this PR uses separate commands for the images so they are scaled appropriately for both formats (60 % one is reduced further than the others to get the placement right across the page breaks, I thought it was still large enough to be legible).