WIP: Reuse license

[MarcelKoch]

This PR changes how we handle our license declaration. It moves from the current block of license (~30 lines) to 3/4 lines as defined by https://reuse.software/. The new license header will now read as:

// SPDX-FileCopyrightText: 2017-2023 The Ginkgo authors
//
// SPDX-License-Identifier: BSD-3-Clause

Additionally, a license is added to all files in our repository. This includes adding a CC-BY-4.0 license for all of our images. Since the images are binary files, an extra file with a .license suffix is created in all cases.

The MatrixMarket files are properly attributed, but since these files should not be modified (see the license section in https://sparse.tamu.edu/about) this is handled in the .reuse/dep5 file. We could also do the same as with the images for those.

Some files have a CC0-1.0 license which basically means no copyright. The files are the *.stderr and *.stdout in the benchmark tests, and the files examples/*/doc/.

Some files have dual license. These are FindHWLOC.cmake (MORSE + our) and external-lib-interfacing.cpp (deal.II + our). I'm currently using an AND clause to combine the licenses, which means that a user must comply with all licenses simultaneously. The alternative is an OR clause, which would mean that the user can choose a license.

For now I've disabled our pre-commit license check, and instead added a github action for that.

Todo

• enable pre-commit check for license again
• figure out the license of the matrices in matrices/test
• switch to CC0 license for files with little originality, eg gitignore, clang-format, etc

Benefits of this PR

• Removes ~30k LOC which is just our license
• Fewer 'ignored' lines in upcoming PRs
• Clearly states the license for all files (no ambiguity)

[upsj]

This is no longer true since AMD was added

[upsj]

Maybe we can do this in two separate PRs? First replace the license by the SPDX header, then handle automation?

[MarcelKoch]

@upsj This PR handles only the license replacement atm. It only disables our current handling, because that doesn't mix well. The automation (if wanted) should best be handled by pre-commit.

But as you noted, we need to adjust the license for files with very little originality.

[upsj]

I mean shortening only the existing license headers, everything else is a bigger effort

[MarcelKoch]

I will split this PR up into two parts:

1. changing only existing license (as @upsj suggested)
2. adding license to other files using the reuse automation.

I will close this PR in favor of the other two.