ontograph, read format from name

[francescalb]

Ontograph no longer takes the format frmo the name.
e.g. the command ontograph --root=Material --relations=all --legend emmo-inferred material.png
produces the file material.pdf and not material.png (Ubuntu on wsl2).
adding the option format=png produces material.png

If format not specified it should tr to guess it from the name of the file.