Refactor gmsh mesh generators

[bmxam]

• I have refactored a bit the gmsh mesh generators (we could go even further);

• I have removed all dependencies of test to input/mesh : I've converted all the gmsh input file into gmsh_utils.jl functions that generate the equivalent meshes. Once the BcubeTutorial repo will be available, there will be no more input folder. For _gen_2cubes_mesh and _gen_pile_mesh, we could directly create the Mesh manually, but having the gmsh script enable the possibility to easily complexify the mesh. @ghislainb what do you think?

• In the test, I systematically create a new gmsh mesh when needed. @ghislainb : would you prefer that we create all the mesh once and for all in the runtests.jl file and provide a list of path to the included files? That will save some time, but it will be a bit less flexible at lower level

• I have updated the readme.md to fix the gallery issue;

• some minor functions signature modifications to improve compatibility with BcubeParallel.

[bmxam]

I am setting it as a draft cause I will add some tests (with the sha1 procedure).