Boost build into stan-math?

Developers
3

The different ways we could do this:

  1. (simplest) just include all of boost in the math distribution
  2. put things that aren’t used by Stan / Math that we need for conditional builds (MPI / GPU) into a separate folder
  3. expect the user to install separately; boost can be installed using things like brew or apt

My preference is to include everything from boost into the math library. I know that means there’s pruning done by RStan and PyStan, but I think that step is already done now. It’ll make it easier for developers if it’s all sitting there.