Implement helicity recycling in our CUDA/C++

[valassi]

This is a followup of issue #276.

In order to compare C++/CUDA and Fortran throughputs, we should make sure that they use the same algorithm. This is presently not the case: we are comparing a faster Fortran with helicity recycling to a slower C++ without helicity recycling.

In issue #276, I will follow up a better estimate of slower Fortran without helicity recycling, that can be directly compared to C++.

But what we really need to do is implement helicity recycling in the CUDA/C++. @oliviermattelaer is this something that would be complicated (an/or maybe is already underway)? Thanks

[oliviermattelaer]

I would actually doubt that doing helicity recycling for gpu is a good idea since this blows up the size of the code and the memory requirement. For vectorised cpu, that is obviously an option.

[valassi]

Within PR #401, note that I had to introduce this fix while moving to v311
04d4b8e
This indeed picks up a new feature of v311 (#360), but it is only relevant to helicit yrecycling (#279).

In practice

• initially I started generating code (madevent only, or madevent+cudacpp) without disabling heliicty recycling
• under this configuration, code generation of madevent+cudacpp did not work, and I had to fix it as above
• this was especially puxxling because I had to change some code generation lines (for generating cudacpp) which were not used in my normal cudacpp-only version
• after fixing it, I got code with helicity recycling, which however did not build (not clear why, but not an issue today): to fix the build I disabled helicity recycling
• in retrospective, the reason why I had to fix those lines is thus for helicity recycling code, which in any case I later decided to disable: if I had disabled helicity recycling before trying to generate madevent+cudacpp code, probably that patch would not hav ebeen necessary