The *hxt* standalone repos that we are maintaining on a private Gitlab
is becoming a total mess IMO. Therefore, as I'm at the end of my
thesis and I want to distribute the *hxt* library in a "clean" manner,
I thought I could maybe clean the *hxt* directory in *contrib*, and
make it so that we can compile things independently. Therefore, I
created this huge commit.

 - Keep a directory structure, even if it is in contrib.
   Each part of *hxt* has its own CMakeList.txt, and is compiled
   by gmsh as an OBJECT library target.
   Each target can be separately compiled.
   For example, you can go in *contrib/hxt/tetMesh* and do
   `mkdir build && cd build && cmake .. && make -j4`
 - Files that were not used by gmsh were deleted.
   Metis was actually not used.
 - tetMesh was updated to the newest version
 - hxt_octree.cpp was almost entirely surrounded by `#ifdef HAVE_P4EST`
   and it was throwing an error if it wasn't the case
   Therefore, I change that so that HAVE_P4EST was a prerequisite for
   the compilation.
 - You might notice cmake_minimum_required in *hxt* is version 3.9
   to enable target_compile_features(... c_std_99)
   I doubt that people who wants to have HXT_ENABLED AND
   have a compiler with OpenMP 4+ and C99 AND have the latest version
   of Gmsh will have problem installing a newer version of CMake.