## Summary of Changes
The testsuite of
[Triangulation](https://cgal.geometryfactory.com/CGAL/testsuite/summary-6.1-Ic-159.html?package=Triangulation)
has N's for some platforms. The test platforms have in common to use
boost_1_88_0, but with VC++ or clang on Windows I can't reproduce the
error with this version of boost.
In this PR I use the filter_iterator adaptor of CGAL, to see if the
boost filter_iterator is the problem.
## Release Management
* Affected package(s): Triangulation
* License and copyright ownership: unchanged
## Summary of Changes
Add a new package computing the approximate Fréchet distance between two
polylines in dD under Euclidean metric, or decides if the distance is
smaller than a given value.
## Release Management
* Affected package(s): Frechet_distance
* Feature (if any):
[link](https://cgal.geometryfactory.com/CGAL/Members/wiki/Features/Frechet_Distance)
* License and copyright ownership: MPI, GeometryFactory, cnrs(?)
## TODO:
- [x] check branch size (@sloriot)
- [x] Make it work with `Epick_d<Dimension_tag<2>>` @sloriot
the patches actually correspond to PLC faces,
which must be planar, and do not match the "face patches"
of PMP::isotropic_remeshing for example
+ add a np to CGAL::IO::write_MEDIT(ccdt) to save the corresponding PLC ids,
or just "1" everywhere as patch id
## Summary of Changes
`const` variables declared at global scope have internal linkage, and so
if they are declared in a header file, they are replicated in every file
that `#include` the header. They also can't be the target of `using`
declarations that exports the variable into a C++20 module. This change
marks a number of variables with `inline`, which gives these variables
external linkage, allows the linker to unify all of these variables
between different `.o` files, and also allows for their use in C++20
modules.
There is a separate question whether these variables could or should be
`constexpr` instead of just `const`. The difference between `constexpr`
and `const` has no effect on linkage, and so in the spirit of taking one
step at a time, this patch does not address the `constexpr` vs `const`
question.
This patch fixes#8871. Related to
https://github.com/dealii/dealii/issues/18071.
## Release Management
* Affected package(s): 5.6.1
* Issue(s) solved (if any): #8871
* License and copyright ownership: Same license as used throughout CGAL.
Mael Rouxel-Labbé
Sébastien Loriot
Laurent Rineau
albert-github
Jane Tournois
Andreas Fabri
Léo Valque
Wolfgang Bangerth