Draft: Tuto obstacle

Adds a tutorial case with a simple, smooth obstacle and a ready-to-go configuration.

specific/configuration/green0_preconditioner.h

@@ -13,6 +13,7 @@
 #include "line_acquisition.h"
 #include "rectangular_acquisition.h"
 #include "circular_acquisition.h"
+#include "flexible_acquisition.h"
 
 using namespace gmshfem;
 using namespace gmshfem::common;
@@ -44,6 +45,10 @@ ModelMonoFunction green0_preconditioner(const Data<T_Physic>& dd, const WaveMult
     {
         return circular_acquisition::green0_preconditioner(dd,g,circular_acquisition_config);
     }
+    else if (const flexible_acquisition::Configuration* const flexible_acquisition_config = dynamic_cast<const flexible_acquisition::Configuration* const>(config))
+    {
+        return flexible_acquisition::green0_preconditioner(dd,g,flexible_acquisition_config);
+    }
     else
     {
         throw Exception("Dynamic cast to specific configuration has failed.");