Buoyancy migration mdpa

[jginternational]

Migration of the buoyancy application

when ready, merge to #985

[jginternational]

@rubenzorrilla I'm storing here an example that is failing
buoy1.gid.zip

Error message:

"C:\Users\garat\Desktop\Deploy_Kratos\Deploy_Kratos\KratosMultiphysics\FluidDynamicsApplication\check_and_prepare_model_process_fluid.py", line 96, in Execute
    if KratosFluid.FluidMeshUtilities.AllElementsAreSimplex(fluid_computational_model_part):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Error: There are no elements in model part 'FluidModelPart.fluid_computational_model_part'.