add the norm of the gradient as output of the optimization of the cost function and improve the use of custom stopping criteria for the optimization using StopIteration exceptions

clean the advection_operator module, the rmatvec method of the Advection class is now the true adjoint matrix-vector product of the advection operator and remove the AdjointAdvection class that is no longer used, change the pheromone propagation model and adjoint model accordingly, as well as the tests

update the solver of the adjoint model, move the former solver method to a more generic method given an adjoint derivative observation operator and specialize a solver for the classical adjoint model and one for the one-sensor adjoint model

update the solver of the adjoint model, move the former solver method to a more generic method given an adjoint derivative observation operator and specialize a solver for the classical adjoint model and one for the one-sensor adjoint model

improve the raise of error when the integration time window for the first observation starts before the temporal domain

fix reference path issue in the notebook and minor improvment of the notebooks and of the tutorials rst file