Vibro-elastic first example

Easy test case of vibro-elastic coupling, with elastic displacement forced to be 1D and a potential formulation on the acoustic side. An incoming acoustic wave comes from top and reaches a perpendicular interface between the acoustic region and the elastic region. Depending on the material properties, some reflection and transmission occur. The code computes the analytical solution (i.e. reflection and transmission coefficients given the material properties) and checks they are reproduced, up to tolerance.

I put it as an example, but in the long run it should probably be put inside some CI, as the code checks correctness explicitly