The old behavior was to call wkhtmltopdf for each report and to put all of them together
using the merge_pdf method. It was a very slow approach because all report needs
5 temporary files, one subprocess and then, a merge.
The new behavior is to perform a single call to wkhtmltopdf by putting all the footers/headers
html together and so, avoid call to merge_pdf to greatly improve the performances of the reporting.
Laurent Smet
b446930dc4