- 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 new version property to handle joined version numbers
* Add unit test for new joined property
* Add documentation on version.up_to() and version.joined
- use two empty lines before `class`
- change version numbering scheme for packages, use `url_for_version` to make things work
- specify dependency types
- add comment about temporarily moved download location
- update two packages to newer versions
