fix: use Constexpr

	-	We can use constexrp here.

Author: sbstndb

include/samurai/mesh.hpp

@@ -919,7 +919,7 @@ namespace samurai
         std::size_t subdomain_end   = 0;
         lcl_type subdomain_cells(start_level, m_domain.origin_point(), m_domain.scaling_factor());
         // in 1D MPI, we need a specific partitioning
-        if (dim == 1)
+        if constexpr (dim == 1)
         {
             std::size_t n_cells               = m_domain.nb_cells();
             std::size_t n_cells_per_subdomain = n_cells / static_cast<std::size_t>(size);
@@ -942,7 +942,7 @@ namespace samurai
                                       }
                                   });
         }
-        else if (dim >= 2)
+        else if constexpr (dim >= 2)
         {
             auto subdomain_nb_intervals = m_domain.nb_intervals() / static_cast<std::size_t>(size);
             subdomain_start             = static_cast<std::size_t>(rank) * subdomain_nb_intervals;