Add consistent file-header comments

[jopperm]

Options:

• Standard Apache v2.0 boilerplate text: https://choosealicense.com/licenses/apache-2.0/ (bottom of page). Requires naming of a copyright holder.
• In the LLVM world, they use the wording "part of the XYZ project" and just refer to the license (see: https://llvm.org/docs/CodingStandards.html#file-headers).

[eyck]

No, we will use SPDX license identifiers: https://spdx.org/licenses/. There is a short notion: https://spdx.dev/ids/ which avoids the boilerplate. A tutorial can be found here: https://github.com/david-a-wheeler/spdx-tutorial/

[jopperm]

Cobbling together [1] and [2], I suggest:

// Copyright 2020 - 2022, MINRES Technologies GmbH and the CoreDSL contributors
// SPDX-License-Identifier: Apache-2.0

[1] https://github.com/david-a-wheeler/spdx-tutorial/
[2] https://ben.balter.com/2015/06/03/copyright-notices-for-websites-and-open-source-projects/