Fix interface when t0 != 0

[schillic]

For a time period t0 to t1, the following is used for TMJets:

dt = t0 .. t1
post(alg_TMJets, ivp, dt)

This works because TMJets reads out t0 from dt.

Other algorithms only read out the final time and instead a separate argument Δt0 for the offset must be passed:

t_span = zero(t0) .. (t1 - t0)
t_offset = interval(t0)
post(alg_other, ivp, t_span; Δt0=t_offset)

This interface is incompatible (one or the other crashes when the algorithm is flipped). We must fix this to be able to use different algorithms easily.