[i3_to_json] harmonize extra particle extraction

[mlincett]

Quoting from the current README (following #15):

It is not possible to copy an arbitrary key/object from the input P-frame to the output P-frame. There is also no warranty that a particle extracted with the --extra option will have a corresponding object in the output P-frame. If you need such a feature, or you want to improve this behavior, feel free to file a pull request!

Ideally:

• it should be possible to write out arbitrary keys even if they are not I3Particle objects;
• all objects corresponding to I3Particle-s extracted with --extra should be written in the output P frame.

None of these should be particularly hard to implement but I did not want to further delay the merge of #15