Fix ti, tf variable error. Add behavior session as argument

[AdrianRoggenbach]

There was a small error in the arguments of the plot_stimuli function, which does not result in an error, but the input arguments to the function do not change the range of the plot.

Additionally, I added behavior_session to the arguments to make the functions independent of the variable assignment of behavior_session when the function was defined. This makes the code more modular and reusable.

[galenlynch]

@AdrianRoggenbach the website should now be updated with your changes!