move function knit_to_pdf to utils.R
move function knit_to_pdf to utils.R

fboehm/cboettig_site,fboehm/labnotebook,fboehm/cboettig_site,fboehm/labnotebook,fboehm/cboettig_site,fboehm/labnotebook,antass/labnotebook3,antass/labnotebook3,antass/labnotebook3,fboehm/cboettig_site,fboehm/labnotebook,fboehm/labnotebook,fboehm/cboettig_site,fboehm/labnotebook