make get_mean_values suport t_last and t_start for tmax and tmin + black

Modify fluidsim.base.output.base.get_mean_values such that we can pass the following argument:

tmin="t_start+?"
tmin="t_last-?"
tmax="t_start+?"
tmax="t_last-?"

This is useful when we want to compute the mean values over the last (or the first) times only for example.