I am new to Nektar++ and I would like to ask you some questions which I couldn't find in the documentation.

- Plotting streamlines: I have been trying to plot streamlines with Paraview; any additional output option must be set in the .xml file?

- Adding tracers: is there the possibility to add tracers to the simulations? That is, a way to track particles or groups of them throughout the simulation

- Probes: is there a way to put probes? i.e. to output the change of certain variables at a particular computational domain's point.

Yep we call these history points. Details of this should be in the user-guide. 

- Iteration curves: is there a way to plot the residuals (velocity's, for example) vs iterations? How to output this?

Not quite sure what you require here but if you run with the -v option you can see the number of iterations per time step. 

- SFD: I would need to have a look at the source where this method is defined but the search option of the doxygen-generated documentation does not work. Which file should I have a look at?

You should have a look at the Driver files. There are in the library 
Also it would be good if you can send these request to the nektar-user list  since you may then get more helpful responses from others.

Cheers,

Spencer.