- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
* new package to be added: NuT
* removed cmake_args() and made other changes as suggested
* Added setup_environment()
* PEP8 compliant
* edited version name, build_targets, and files/directories to install
* corrected executable directory
* compiler dependencies based on version
