Add APPLgrid importer

[cschwan]

We need a subcommand in pineappl which is able to convert APPLgrids into PineAPPL grids.

[cschwan]

Commits c8f8b66, ae6bdbd, and 49d9c8b add a preliminary C++ program to convert APPLgrids into PineAPPL grids. I've only tested a single grid, which is NNPDF's single-top production grid CMS_SINGLETOP_TCH_R_7TEV. The program only works with the NNPDF-modified version (also used for FK table generation) of APPLgrid, which exposes internals that are needed for the conversion.

To do list:

• test with more APPLgrids, especially ones generated by SHERPA (so far only one aMCfast grid has been tested)
• the reweighting parameter is only guessed; we now try out both values
• the order of the electroweak coupling constant should be given by the user, currently it's set to zero (doesn't hurt)
• ideally the program is written in Rust and is a subcommand of pineappl

[cschwan]

For the grids that are not working, check if a map different than "f2" is used (?!).

[cschwan]

Commit 53b48c4 massively improves the converter. I've checked it against all the APPLgrids in the NNPDF repository. There are 491 *.root files in total, out of which

• 273 succeed to convert and yield the same numbers (at least 10 digits, typically more) and
• 218 fail to convert.

Out of the 218 files that fail to convert,

• 125 have luminosities functions different from lumi_pdf which currently are not handled; fixed in commit 0056e50,
• 1 grid has a luminosity function different from lumi_pdf, and two bins combined into a single one, which makes the final check fail; this can't be fixed without change the APPLgrid header (there's no read access to the combination vector),
• 87 convert, but the numbers are different; fixed in commit 5b0f1b4,
• 4 terminate with an LHAPDF::RangeError (x is large than 1); fixed in commit f030e03 and
• 1 grid (ATLAS1JET11/nlojet++_ATLAS1JET11/obsLimits.root) terminates with an exception: grid::grid() cannot get tags; this is an exception when loading the ROOT file, so there's little we can do; this probably isn't an APPLgrid, but rather a ROOT file.

[cschwan]

For the 87 grids that can be converted but give wrong numbers the reweighting must be disabled.

[cschwan]

I'm closing this issue, given that almost all grids can be successfully converted. The program has to used as follows:

./appl2pine grid.root output.pineappl

It must be compiled with APPLgridphoton.