Update dependencies and streamline installation

[marcelo-alvarez]

Remove Boost and other external dependencies no longer needed for the DESI software stack.

[sbailey]

A few additional changes I made:

• don't set SLURM_CPU_BIND=cores by default; leave it to desispec batch wrappers to opt-in when needed
  • reason: although it helps pure MPI codes a little, it hurts MPI+multiprocessing codes a lot, so make the default a "ok performance for everyone" instead of "a bit better for some and terrible for others"
• documentation tweaks to make the installation cut-and-paste from the README
• added scripts/boostrap-desi.sh for the steps to install modules for the master branches of all the desi packages

I tested this with installation + dark + nightlybias + psf and desispec unit tests. Full end-to-end was tests with similar earlier versions of this branch. I'll merge now and then proceed with making a reference tag+installation.