`stroboscopicmap` should allow configuring a `t0` argument

[Datseris]

Just use by default ds.t0 as a keyword. Pass directly to the generated integrator underlying the evolution.

[Datseris]

Actually, only referncing this is not good enough. We should also add a simple test of generating two stroboscopic maps of the duffing, each with different t0, and getting their attractors. Compare attractors and ensure they are different.