Added machinery for creating metrics report in PDF format

Besides creating an Excel file, the metrics module will create a PDF file as well. Just like for the Excel file, this routine creates a PDF in a TMP directory which will be retrieved when people click on the button to retrieve the PDF report. Just like for the Excel files, PDF files older than 2 hours will get removed. The PDF creation uses the module 'reportlab' (added to requirements.txt). Using version 2.7 is a requirement, because the most recent version only works with Python 2.7.