New tutorial case: overlapping DDM (Dirichlet) for Laplace equation

Added a tutorial to solve a Laplace equation with overlapping DDM. (x is in [0,2]. First domain solves on [0, 1.2] whereas second domain solves on [0.8, 2]) It's a more basic tutorial than the current ones, so it's probably a good introduction to the library.

Not 100% sure my implementation is optimal. In particular, is my weak imposition of continuity ideal ?