Meshes: use markers to define FESTIM subdomains for better readability

in [Mesh with GMSH](https://festim-workshop.readthedocs.io/en/latest/content/meshes/mesh_gmsh.html#festim-model-setup), currently subdomains are defined using raw numerical value for the id:
`tube_surf = F.SurfaceSubdomain(id=7)`

It makes it non-obvious from a learner perspective that this is the way we are mapping the subdomains to their location in the mesh.
-> use the marker variables defined above (`inlet_marker`, `outlet_marker`,...) directly instead of their numerical value:
`tube_surf = F.SurfaceSubdomain(id=obstacle_marker)`