Add SPDX identifiers to all files
Describe the feature you would like to be implemented.
I would like to see SPDX license identifiers in all files in the Eigen repository.
Would such a feature be useful for other users? Why?
SPDX identifiers are a standardized way to identify open-source licenses, making it easier to understand and communicate licensing information. By clearly marking the license in each file, SPDX identifiers make it easier for others to reuse the code while ensuring they comply with the license terms. Automated software tools can also use SPDX license identifiers to maintain an updated list of licenses used in software that uses Eigen.