[T/NL] Fix mesh_length type to double.

Fixes Tests\NumLib\TestComponentNorms.cpp(39): warning C4244: 'argument': conversion from 'const size_t' to 'const double', possible loss of data

Tests/NumLib/TestComponentNorms.cpp

@@ -48,10 +48,10 @@ struct DOFTableData
     NumLib::LocalToGlobalIndexMap const dof_table;
 
 private:
-    static const std::size_t mesh_length;
+    static const double mesh_length;
     static const std::size_t mesh_elements_in_each_direction;
 };
-const std::size_t DOFTableData::mesh_length = 1;
+const double DOFTableData::mesh_length = 1;
 const std::size_t DOFTableData::mesh_elements_in_each_direction = 5;
 
 template <typename AccumulateCallback, typename AccumulateFinishCallback>