Open pdf after compiling (configurable)
Adress comments

 * move file extension addition outside of function
 * move log statement before function call
 * change default to False
 * add postprocessor to nbconvertapp
Merge pull request #4348 from jakobgager/openpdf

Call PDF viewer after latex compiling (nbconvert)

Add a configurable to open the pdf after compiling.
Uses the default application.

closes #4291