Owl_plplot can't set device/driver independently of filename suffix

[ComposMin]

The function _set_device infers which Plplot device/driver to set based on the suffix of the output file. This makes it awkward to use the pdfcairo, pngcairo drivers - as you must use this as the suffix to generate the graph, but then usually need to rename the file to have other standard tools interact with the file.

https://github.com/owlbarn/owl/blob/master/src/plplot/owl_plot.ml#L419-L425

I'm happy to contribute a code fix, but wondered if you have any specific guidance on the shape of the fix, so that it doesn't break existing code.

[ComposMin]

Making use of the Cairo drivers makes it easier to use Owl_plplot running in Jupyter notebooks environments that use Docker (without X11 dependencies).