Escape filename for pdflatex, otherwise filenames with spaces cause issues.
Provide a list of the command and argument to subprocess for PDF post-processor, which is a better way to avoid issues if there are spaces in the filename.
Renamed compiler to command
Merge pull request #3865 from astrofrog/fix-nbconvert-pdflatex-spaces

use `command` list instead of compiler string in PDFPostProcessor

avoids issues with spaces