add operators examples of use

docs/source/usage.rst

@@ -17,6 +17,17 @@ Tutorials
 
 
 
+The different numerical schemes for the differential operators can be tested independently with dedicated tutorials
+
+.. nbgallery::
+    tutorials/test_numerical_scheme/test_advection_operator.ipynb
+    tutorials/test_numerical_scheme/test_diffusion_operator.ipynb
+    tutorials/test_numerical_scheme/test_adjoint_advection_operator.ipynb
+    tutorials/test_numerical_scheme/test_adjoint.ipynb
+    tutorials/test_numerical_scheme/test_EDP_model_adjoint.ipynb
+    tutorials/test_numerical_scheme/test_EDP_model_direct.ipynb
+    tutorials/test_numerical_scheme/test_gradient.ipynb
+
 These tutorials can be found in the folder 'tutorials' of the repository as jupyter notebooks.