Pythia8 support

[EinarElen]

Note: I'm opening this issue now, but there isn't exactly a rush to introduce it. In particular, the exact version of Pythia8 that we would want might need some consideration.

Is your request related to a problem? Please describe.

Pythia8 has a bunch of uses in the future of our software toolchain and is a straight-forward dependency which is mostly self-contained.

Use cases that I know of that we should make sure that we can support

• DM simulation with Pythia8/MG5
  MadGraph5 can generate matrix element code directly for Pythia8 for 2-1, 2-2, and 2-3 processes. Thus, we can directly run simulations based on our current MadGraph setup (if we can port it to MadGraph 5 which I think we can). This should work with any recent version of Pythia8, although we might need some patches to make sure that the 2-3 process generation works well with our somewhat odd 2-3 process with one very heavy initial state particle. Currently being looked into.
• Generating LHE files for other BSM processes that people want to study with LDMX-sw
  This has been requested from UCSB. Could also of course be handled in a separate repo like we do with the MG4 code, although if we already have Pythia8 in the container we might as well use it here. Depending on the needs of the user, this could also benefit from the previous matter
• Potentially for GENIE
  At least according to this comment in the Dockerfile :)

(Ideally GENIE works with Pythia8? But not sure that works yet despite the adverts that it does.)

Describe the solution you'd like. What packages do you want installed?

The only package we need to add to be able to build Pythia8 from source is rsync. Pythia's makefiles use rsync for the make install step. Otherwise the build is a straight-forward thing, roughly as follows

git clone ... 
cd pythia8 
./configure --prefix=wheretoinstall --cxx-common="flags that we want, e.g. -std=c++17" ... 
make 
make install