Bumped filepath to libeigen3

[WgJu]

this should allow everyone to compile the code with a up-to-date linux again

[MaartenBaert]

As far as I know, it is not the intention that users include <eigen3/Eigen/...>, see for example: https://eigen.tuxfamily.org/dox/group__QuickRefPage.html#QuickRef_Headers Instead you should probably use the -I compiler option to specify the correct location, e.g. -I/usr/include/eigen3. The location where eigen is installed is dependent on your linux distribution, so this shouldn't be hard-coded in the source code of the program.