Hxt update

I updated and cleaned HXT and decided to structure files into modules for more clarity.

At first, I wanted to compile hxt by creating a cmake OBJECT library target for each modules, and linking with gmsh as if they were external library. This commit explains it all... and the corresponding code in the main CMakeLists.txt

It worked well, except that I wanted features that required cmake 3.9+, and the cmake version used by the window64_ci pipeline was lower. And, apparently, t13.geo fails if we do not have hxt enabled (this is maybe an issue ?).

Therefore, I reverted back to the old bad strategy of having a unique CMakeLists.txt file in hxt that updates global variables.

I however kept the CMakeLists.txt files in each submodules, so that hxt can be compiled independently.