Expose `xif` and `xir` in `PyGrid::convolute_eko`

[felixhekhorn]

we need to expose the SV ratios here
https://github.com/N3PDF/pineappl/blob/101cbdefdd3b9f6c0427de48e57bca53e0f3f843/pineappl_py/src/grid.rs#L406
to make them accessible here: NNPDF/pineko#6