Fix Eigen CMake variable

[valgur]

The upper-case EIGEN3_FOUND variable is available on v3.3.x, but has been removed in newer Eigen versions.

Since Eigen is always packaged with an Eigen3Config.cmake and CMake sets <PackageName>_FOUND automatically for CONFIG-type packages, Eigen3_FOUND should be used instead. From CMake docs:

A set of variables which provide package status information are also set automatically when using a config-file package. The <PackageName>_FOUND variable is set to true or false, depending on whether the package was found.

On an unrelated note - is there a reason why the automatically downloaded Eigen is v3.3.9 instead of v3.4.0?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.70%. Comparing base (3f61cb0) to head (db669ef).

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #48 +/- ## ======================================= Coverage 93.70% 93.70% ======================================= Files 36 36 Lines 779 779 Branches 74 74 ======================================= Hits 730 730 Misses 49 49 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[koide3]

Thanks a lot. I merge it now.

On an unrelated note - is there a reason why the automatically downloaded Eigen is v3.3.9 instead of v3.4.0?

I picked a random stable version I downloaded in the past for no reason, but I now think we should use the latest one. I'll update CMakeLists soon.