Non existing link to open numerical library

[albert-github]

In Documentation/doc/Documentation/Usage.txt(Manual/usage.html) there is a link to:

http://alice.loria.fr/index.php/software/4-library/23-opennl.html

but this link does not exist anymore.

[afabri]

We should link to https://github.com/BrunoLevy/geogram/wiki/OpenNL

[albert-github]

I got the impression that the reference is to the OpenNL library, isn't this a library on top of OpenNL and not the OpenNL library itself?

[afabri]

We have forked it some time ago. It is here. So the link is a link to where the official OpenNL is now.

[lrineau]

Why does CGAL still use OpenNL, instead of Eigen? If CGAL_EIGEN3_ENABLED is defined, it seems that OpenNL is no longer used.

In my opinion, OpenNL could be removed completely from CGAL.

[afabri]

Here it is hard coded.
We could probably replace it with a least squares solver from Eigen.

[afabri]

In fact almost everything can be removed. There is a solver class that would remain but it calls the solver of the traits class which by default is the one using Eigen. However it might make sense to do a benchmark for the two.

[afabri]

When using Eigen it is 10 times faster. Also as there is a missing trailing underscore here that not even compiles in case Eigen was not enabled.