Use Cgal and Eigen3 cmake target

[VincentRouvreau]

This PR is required by future CGAL 6.0

• Factorize add_executable and target_link_libraries with CMake targets in a new function add_executable_with_targets
• Use Cgal and Eigen3 cmake target - requires CGAL >=5.1 and Eigen >= 3.3
• Remove project from each CMake (just keep the main one) as not mandatory and it does not add any functionality

questions :question:

* [ ] Do we update CGAL_VERSION_NR and EIGEN_VERSION_AT_LEAST check in code ?

• [X] find_package(CGAL 5.1.0) does not work with CGAL master (but 6.0.X). Do we open an issue on CGAL side ? https://github.com/CGAL/cgal/issues/8192

[mglisse]

Remove project from each CMake (just keep the main one) as not mandatory and it does not add any functionality

Does it not even help if we have tests with the same name in 2 subprojects, or something like that? Anyway, whatever you prefer.

Do we update CGAL_VERSION_NR and EIGEN_VERSION_AT_LEAST check in code ?

I would say no. The code still works with older versions, it is only the cmake stuff that breaks.

find_package(CGAL 5.1.0) does not work with CGAL master (but 6.0.X). Do we open an issue on CGAL side ?

Yes please. I remember discussing this issue a while ago, but I don't know with whom... If they don't want to change it before the 6.0 release, we will have to handle that differently in gudhi.

[VincentRouvreau]

Does it not even help if we have tests with the same name in 2 subprojects, or something like that? Anyway, whatever you prefer.

From what I read here:

To start a project, we use the project() command to set the project name. This call is required with every project and should be called soon after cmake_minimum_required(). As we will see later, this command can also be used to specify other project level information such as the language or version number.

As all of our code managed by CMake is C++, with the same gudhi version number, cmake minimal version is the same for all the library (easier to maintain),... I prefer to keep one project.

[VincentRouvreau]

Sorry about that bad merge... I start again a new PR from 96f7508 and I close this one. Forget the tensorflow proposal, it will come on a new PR