callback to get the desired element size, (on a sphere of radius 1) at one point on this sphere
* mesh_size_min : float
minimum value of the mesh size (approximatively the minimum value returned by mesh_size) on a sphere of radius 1
* output : string
name of the output file
* projection : string
description of the projection used in the output file in the libproj "+" format (the mesh itself is generated on the sphere)
* boundary_name : dictionary of (int, string)
to assign physicals names to the boundary elements of the mesh, a field (of type integer64) called "entity" should be present in the shape files. Then this parameter can be used to map the values of this field to physical names