Improve octave helpers

src/plot_displacement.m

 close all;
 
-plot(1:length(A),A(:,2));
-axis tight;
+plot(1:length(A),1e-6*A(:,2));
+
+xlabel timesteps
+ylabel displacement

src/plot_neumann.m

 close all;
 
 plot(1:length(A),A(:,3));
-axis tight;
+
+xlabel timesteps
+ylabel traction

src/plot_state.m

 close all;
 
-plot(1:length(A),A(:,1));
-axis tight;
+semilogy(1:length(A),exp(A(:,1)));
+
+xlabel timesteps
+ylabel state