Consistent naming: library vs. command line

[mportesdev]

Currently, the library API and the command-line app use different names for the parameter used to specify the output file. (pdf_file and output-file, respectively)

It should be considered for better consistency to use the same name in both cases, e.g. change pdf_file to output_file in the signature of both PictureShow.save_pdf and pictures_to_pdf in the next minor version.