Add license identifier to each file in the repository

[mayrmt]

Description

MIRCO is licensed under an MIT license. While it is technically sufficient to put the license text into a file in the root directory of the repository, which we have done, it might be nice to include a brief reference to the license into each file in the repository.

Possible Solution and Definition of Done

• Start each file with a license header

  // SPDX-License-Identifier: MIT
  // See LICENSE file in the project root for details.

• Adapt the license header accordingly for non-C++ files
• Add a check to the pre-commit hooks to ensure that all files have such a license statement (see 4C for some inspiration)