Unvendor Eigen and bump to 3.4.1

[dalcinl]

• Unvendor Eigen by removing thirdparty/eigen
• Bump Eigen to 3.4.1 via CMake/FetchContent

[dalcinl]

@rdbisme What a great PR to bump my GitHub stats!

[dalcinl]

A few comments about FetchContent and its relation to find_package.

• For more recent CMake versions (>=3.14), the FetchContent approach interoperates with find_package. If Eigen is properly installed in the system, the system installation will be used.
• For older CMake versions back to 3.14 (when FetchContent was added), the dependency is always fetched from git.

I bumped the CMake requirement to 3.15, one more than the strict minimum for FetchContent, to accommodate for scikit-build-core requirements . If we want to enforce FetchContent/find_package interoperability to always honor system installations, then we need to bump the CMake requirement to 3.24.

[rdbisme]

If we want to enforce FetchContent/find_package interoperability to always honor system installations, then we need to bump the CMake requirement to 3.24.

I think we might want to do this aswell.