Documentation: writing mesh tutorial

## Mesh configuration

The [Mesh](@ref TNL::Meshes::Mesh) class template is configurable via its first template parameter, `Config`.

By default, the \ref TNL::Meshes::DefaultConfig template is used.

Alternative, user-specified configuration templates can be specified by defining the mesh configuration as the `MeshConfig` template in the [MeshConfigTemplateTag](@ref TNL::Meshes::BuildConfigTags::MeshConfigTemplateTag) build config tag specialization.

For example, here we derive the `MeshConfig` template from the [DefaultConfig](@ref TNL::Meshes::DefaultConfig) template and override the `subentityStorage` member function to store only those subentity incidence matrices, where the subentity dimension is 0 and the other dimension is at least $D-1$.

Hence, only faces and cells will be able to access their subvertices and there will be no other links from entities to their subentities.

```cpp

// Define the tag for the MeshTypeResolver configuration

struct MyConfigTag {};

namespace TNL {

namespace Meshes {

namespace BuildConfigTags {

// Create a template specialization of the tag specifying the MeshConfig template to use as the Config parameter for the mesh.

template<>

struct MeshConfigTemplateTag< MyConfigTag >

{

   template< typename Cell,

             int WorldDimension = Cell::dimension,

             typename Real = double,

             typename GlobalIndex = int,

             typename LocalIndex = GlobalIndex >

   struct MeshConfig

      : public DefaultConfig< Cell, WorldDimension, Real, GlobalIndex, LocalIndex >

   {

      template< typename EntityTopology >

      static constexpr bool subentityStorage( EntityTopology, int SubentityDimension )

      {

         return SubentityDimension == 0 && EntityTopology::dimension >= Cell::dimension - 1;

      }

   };

};

} // namespace BuildConfigTags

} // namespace Meshes

} // namespace TNL

```

## Public interface and basic usage

The whole public interface of the unstructured mesh and its mesh entity class can be found in the reference manual: \ref TNL::Meshes::Mesh, \ref TNL::Meshes::MeshEntity.

## Parallel iteration over mesh entities

## Writing a mesh and data to a file