Load `t0` from orbit file

Currently, the tool does not load the t0 parameter from the orbit file, such that a file generated for e.g. the esa-orbits.h5 is incompatible with a simulation run with the same orbit file. This can be circumvented by manually setting t0 to match the orbit file before computing the GW response, but it would be convenient to automatically load that value to match the file.