Add png/jpg writing option

[Hboni]

As mentioned in #46, the support of png/jpg image types can be great, but it has to be possible to save data into those image types.
We can add a parameter into the save node that let the user select the output type of image wanted, but also select automatically the type as the same type as the input file (considering only one type of image in inputs).

[jeaunrg]

fixed with [#51]
But if the image contains infinite value (like nan or inf) we cant save the raw image as .png or .jpg (but it can be possible as .tif I think)