Use Eigen CMake target

[christophfroehlich]

I changed the CMake file to use Eigen's CMake target as described here https://eigen.tuxfamily.org/dox/TopicCMakeGuide.html

See ros-controls/control_toolbox#178 for the issues with RHEL 8.

[saikishor]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.16%. Comparing base (9f7e9e9) to head (7f14732).
Report is 29 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1058       +/-   ##
===========================================
+ Coverage   47.71%   72.16%   +24.44%     
===========================================
  Files          41       41               
  Lines        3871     3639      -232     
  Branches     1833     1781       -52     
===========================================
+ Hits         1847     2626      +779     
+ Misses        751      700       -51     
+ Partials     1273      313      -960     

Flag	Coverage Δ
unittests	72.16% <ø> (+24.44%)	⬆️

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see 34 files with indirect coverage changes

[fmauch]

+1 for using modern CMake :-)