Add PMPSizingField concept to docs

Polygon_mesh_processing/doc/Polygon_mesh_processing/Concepts/PMPSizingField.h

@@ -0,0 +1,59 @@
+/// \ingroup PkgPolygonMeshProcessingConcepts
+/// \cgalConcept
+///
+/// The concept `PMPSizingField` defines the requirements for the sizing field
+/// used in `CGAL::Polygon_mesh_processing::isotropic_remeshing()` to define
+/// the target length for every individual edge during the remeshing process.
+
+class PMPSizingField{
+public:
+
+/// @name Types
+/// @{
+
+/// Vertex descriptor type
+typedef unspecified_type vertex_descriptor;
+
+/// Halfedge descriptor type
+typedef unspecified_type halfedge_descriptor;
+
+/// 3D point type
+typedef unspecified_type Point_3;
+
+/// Polygon mesh type
+typedef unspecified_type PolygonMesh;
+
+/// Numerical type
+typedef unspecified_type FT;
+
+/// @}
+
+/// @name Functions
+/// @{
+
+/// called to check whether the halfedge 'h' is longer than the target edge size
+/// and as such should be split. If the halfedge is longer, it returns the squared
+/// length of the edge.
+boost::optional<FT> is_too_long(const halfedge_descriptor h,
+                                const PolygonMesh& pmesh) const;
+
+/// called to check whether the halfedge with end vertices 'va' and 'vb' is longer
+/// than the target edge size and as such should be split. If the halfedge is longer,
+/// it returns the squared length of the edge.
+boost::optional<FT> is_too_long(const vertex_descriptor va,
+                                const vertex_descriptor vb) const;
+
+/// called to check whether the halfedge 'h' should be collapsed in case it is
+/// shorter than the target edge size
+boost::optional<FT> is_too_short(const halfedge_descriptor h,
+                                 const PolygonMesh& pmesh) const;
+
+/// called to define the location of the halfedge 'h' split in case 'is_too_long'
+/// returns a value
+Point_3 split_placement(const halfedge_descriptor h,
+                        const PolygonMesh& pmesh) const;
+
+/// @}
+};
+
+