We'd like to use a consistent checksum scheme everywhere so that we can:
a) incorporate archive checksums into our specs and have a
consistent hashing algorithm across all specs.
b) index mirrors with a consistent type of checksum, and not one that
is dependent on how spack packages are written.
- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256
- 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
* add OctavePackage
1. remove import CudaPackage which is not needed anymore
2. mention CudaPackage and OctavePackage in packaging guide
3. adjust OctavePackageTemplate
4. add clue file for Octave build
5. sanity check on self.prefix
* use setup_environment
