PpCalculation: Allow PLOT.output_format to be specified

[sphuber]

Currently it is blocked as it is automatically set by the plugin. However, it makes sense that the user may want another format, so why block them from overriding a default set by the plugin?

The limitation was probably added because then the PpParser can expect a consistent .cube format and parse the content. But I think we can just add an exception in the parser and if the file is not in an expected format, we don't parse it at all, but at most just attach as a SinglefileData node.

[mbercx]

I think @ramirezfranciscof also ran into this limitation, or something similar? Could be related to #656, but I still have to look at the specifics.