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 ✅

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

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Additional details and impacted files

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

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[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.