Introduce more general convolution types

[t7phy]

This PR aims to make it possible to optimize grids that perform a double convolution between unpolarized and polarized PDFs or PDFs and FFs by not only checking the PID of the convolution particle but also the convolution type. The grids will require the following new keys:

convolution_particle_1: <PID>
convolution_type_1: PDF/FF/polPDF/polFF
convolution_particle_2: <PID>
convolution_type_2: PDF/FF/polPDF/polFF

[cschwan]

Great @t7phy, thank you very much for this. I will review it as soon as possible!

[cschwan]

I thought about this over the weekend and I think we should refactor this:

1. Introduce a new enum:

   #[derive(Eq, PartialEq)]
   enum Convolution {
      UnpolPdf(i32),
      PolPdf(i32),
      UnpolFf(i32),
      // we can add more types as they are needed
   }

2. Write the method:

   impl Grid {
      pub fn convolutions(&self) -> Vec<Convolution> {
           todo!()
      }
   }

3. Use the method from 2) in symmetrize_channels.

[t7phy]

what is the 'convolutions' function supposed to do actually?

[cschwan]

what is the 'convolutions' function supposed to do actually?

It should return a Vec with the convolution types for the grid, so for a proton-proton grid we'll return vec![UnpolPdf(2212), UnpolPdf(2212)].

[cschwan]

@t7phy I simplified the implementation a bit in commit cc7fd091b01e355b606cf0d16233ac9a45a3333f. What's missing is the following:

• [x] an exception for the old case: if a metadata initial_state_1 = id is set, and the channel has that same id set everywhere in the right place, then Convolution::None should be returned. The is necessary to not break the DIS grids which have the PDG ID of a lepton in one of the convolutions
• [x] remove the methods Grid::has_pdf1, Grid::has_pdf2, Grid::initial_state_1 and Grid::initial_state_2 and use the new method Grid::convolutions instead; added in commits de6d9a3, 3a9a787, e232e67 and d7fc831
• [x] add a new method Grid::set_convolution(usize, Convolution) used to set the convolution and use it everywhere initial_state_1 and initial_state_2 was directly set; added in commit d7fc831
• [ ] whenever we use grid.convolutions()[0] or grid.convolutions()[1] we should make sure to prefix the code with assertion statement assert_eq!(grid.convolutions().len(), 2); or make the code generic for every length
• [ ] write some small unit tests