Add SPDX license identifier for files that are under MPL-2.0 license
Reference issue
What does this implement/fix?
This patch adds SPDX license identifiers to most files under an MPL-2.0 license.
Additional information
The SPDX license identifier makes it easier to reuse Eigen by declaring each file's license in a human and machine-readable way. This can, for example, be used to generate a software bill of material for a product automatically.