Set Eigen version to 3.4.1 or 4.0.0 once it is released

[whaeck]

Eigen 3.4.0 was released in August of 2021. While the library has been working perfectly fine for us, there is one little issue that has proven to be quite annoying: Eigen's unit tests are registered with ctest unless we set BUILD_TESTING to OFF because Eigen 3.4.0 does not have an Eigen specific BUILD_TESTING guard. This is actually not something one wants to do (since BUILD_TESTING is the global cmake variable for testing).

However, the 3.4.1 branch (not a tag, this is important) has been updated in July of 2023 with a cmake fix that resolve this. Scion currently uses the July 26, 2023 commit that made this change instead of the original 3.4.0 tag. Once either the 3.4.1 or a higher verison of Eigen is released, we should move to that release as soon as possible.

This issue is opened as a reminder to check for a new Eigen release from time to time. Once a new version of Eigen is released, we can close this issue.