Internal headers are no longer allowed as of
https://gitlab.com/libeigen/eigen/-/merge_requests/631. Based on
benchmarking I conducted in that thread, there doesn't seem to be a
performance penalty for including the full headers anymore.
This helps reduce compilation overhead. I tried slimming down includes
of <Eigen/QR>, but the householderQr() function we use from there
requires including dependency headers from Eigen that don't fit with
lexographic ordering. It didn't seem worth the effort to work around.
This won't affect user code at all since all the Eigen feature usage
here is internal only; users generally only need <Eigen/Core>.
