Exposes more functions to C-API for APFEL++

[Radonirinaunimi]

Exposes more functions to the C-API for APFEL++, as discussed at DIS, in order to achieve two things:

• Interface APFEL++ with PineAPPL in order to get (SI)DIS and SIA predictions (everything is already available)
• Provide APFEL++ with an interface to read the contents of a PineAPPL grid and perform the evolution on the fly. Various ingredients are needed for this:
  • functions to get and rotate the particle ID representation
  • function that returns the list of channel combinations (see display-channels.cpp)
  • function that returns the contents for the subgrids for a given bin, order, and channel (see get-subgrids.cpp)

cc @vbertone, @enocera

[Radonirinaunimi]

Perfect! Thanks @cschwan!

[cschwan]

@Radonirinaunimi can you please write another small test case, in which you call pineappl_grid_subgrid_array on an empty subgrid? The call is enough, and the test should make sure it doesn't panic/abort.

[Radonirinaunimi]

@Radonirinaunimi can you please write another small test case, in which you call pineappl_grid_subgrid_array on an empty subgrid? The call is enough, and the test should make sure it doesn't panic/abort.

Yes, absolutely! I will write one this evening.

[Radonirinaunimi]

Actually, just did it now in 7797940, as it was just 2 lines.

[cschwan]

@Radonirinaunimi please have a look at commit 9d6f897. If you think that makes sense, feel free to merge!

[Radonirinaunimi]

@Radonirinaunimi please have a look at commit 9d6f897. If you think that makes sense, feel free to merge!

Yes, that indeed makes perfect sense, thanks! I will merge once the test are done.