[DOC] Reduce example computation time

[emdupre]

Current computation time estimates for all examples :

example	time elapsed
plot_template_alignment.py	1442.31 sec
plot_int_alignment.py	118.17 sec
plot_alignment_methods_benchmark.py	85.85 sec
plot_pairwise_alignment.py	82.16 sec
plot_pairwise_roi_alignment.py	19.88 sec
plot_toy_int_experiment.py	5.61 sec
plot_alignment_simulated_2D_data.py	1.79 sec

We should bring all of these down, but particularly plot_template_alignment !