Add free boundary condition descriptor

The aim of this kind of boundary condition descriptor is to indicate
that as "special" treatment is to be performed for a boundary that
is not subject to a boundary condition at the continuous level.

For instance it may occur when using a centered scheme to discretize a
system which has only one wave (quite often with scalar laws:
transport, traffic flow,...)

It can be used both to change locally the scheme and to check that no
boundary has been forgotten in the description

src/language/modules/SchemeModule.cpp

@@ -6,6 +6,7 @@
 #include <mesh/Mesh.hpp>
 #include <scheme/DirichletBoundaryConditionDescriptor.hpp>
 #include <scheme/FourierBoundaryConditionDescriptor.hpp>
+#include <scheme/FreeBoundaryConditionDescriptor.hpp>
 #include <scheme/IBoundaryConditionDescriptor.hpp>
 #include <scheme/IBoundaryDescriptor.hpp>
 #include <scheme/NamedBoundaryDescriptor.hpp>

src/scheme/FreeBoundaryConditionDescriptor.hpp

+#ifndef FREE_BOUNDARY_CONDITION_DESCRIPTOR_HPP
+#define FREE_BOUNDARY_CONDITION_DESCRIPTOR_HPP
+
+#include <scheme/IBoundaryConditionDescriptor.hpp>
+#include <scheme/IBoundaryDescriptor.hpp>
+
+#include <memory>
+
+class FreeBoundaryConditionDescriptor : public IBoundaryConditionDescriptor
+{
+ private:
+  std::ostream&
+  _write(std::ostream& os) const final
+  {
+    os << "free(" << *m_boundary_descriptor << ")";
+    return os;
+  }
+
+  std::shared_ptr<const IBoundaryDescriptor> m_boundary_descriptor;
+
+ public:
+  const IBoundaryDescriptor&
+  boundaryDescriptor() const
+  {
+    return *m_boundary_descriptor;
+  }
+
+  Type
+  type() const final
+  {
+    return Type::free;
+  }
+
+  FreeBoundaryConditionDescriptor(std::shared_ptr<const IBoundaryDescriptor> boundary_descriptor)
+    : m_boundary_descriptor(boundary_descriptor)
+  {
+    ;
+  }
+
+  FreeBoundaryConditionDescriptor(const FreeBoundaryConditionDescriptor&) = delete;
+  FreeBoundaryConditionDescriptor(FreeBoundaryConditionDescriptor&&)      = delete;
+
+  ~FreeBoundaryConditionDescriptor() = default;
+};
+
+#endif   // FREE_BOUNDARY_CONDITION_DESCRIPTOR_HPP

src/scheme/IBoundaryConditionDescriptor.hpp

@@ -10,6 +10,7 @@ class IBoundaryConditionDescriptor
   {
     dirichlet,
     fourier,
+    free,
     neumann,
     symmetry
   };