Simplified setup, not working yet

developement_help.md

@@ -3,9 +3,23 @@
 - add it to README.md
 - add the necessary Rules in ```utils.parameters```
 - optional : add a default value
-- optional : add to valid_variable
 - optional : add to mandatory_parameters
 
 ## complicated Rule conditions
 - add the desired parameters to the init of the operator
-- raise OperatorError if the conditions are not met
+- raise OperatorError if the conditions are not met
+
+## operators
+There are 3 kinds of operators
+- `SpecOperator(spec: np.ndarray, z: float) -> np.ndarray` : operate on the spectrum
+    - Envelope nonlinear operator used in the solver
+    - Full field nonlinear operator used in the solver
+- `FieldOperator(field: np.ndarray, z: float) -> np.ndarray` : operate on the field
+    - SPM
+    - Raman
+    - Ionization
+- `VariableQuantity(z: float) -> float | np.ndarray` : return the value of a certain quantity along the fiber depending on z
+    - dispersion
+    - refractive index
+    - full field nonlinear prefactor
+    - nonlinear parameter (chi3, n2, gamma)