Install boost headers only

[climbfuji]

At JCSDA we only use the boost headers. As of today, there is no option (variant) in spack for this, therefore I made the boost installation a headers-only one by turning off virtually everything:

    boost:
      version: [1.72.0]
      # Make this a boost-headers package by compiling basically nothing
      variants: ~atomic ~chrono ~date_time ~exception ~filesystem ~graph ~iostreams ~locale ~log ~math ~mpi ~numpy +pic ~program_options ~python ~random ~regex ~serialization ~signals ~test ~timer ~wave cxxstd=14 visibility=hidden

We need to check if there is a better way to do this. And/or use the module projections to rename the boost module to boost-headers to avoid any misconception.

[kgerheiser]

I looked into this the other day and I don't think there is a better method, but using projections to change it is easy.

[climbfuji]

Issue #49 requires adding ecflow to spack-stack. ecflow requires a boost installation with about half the libraries being built, so this issue is no longer relevant.