Add possibility to query (x,Q) arrays in the C/Fortran examples

[scarrazza]

As the title says.

[scarlehoff]

As I said earlier, don't really have the appetite (or incentives) to work on this but in this branch there's a minimal example that does that: https://github.com/N3PDF/pdfflow/tree/vectorized_example

If someone is interested enough on this such that they would like me to make something more palatable (or even usable gasp), I'd be happy to oblige, but for the time being there's that.