Fix starting time when interpolating frequency plan files

The frequency plan files are currently always interpolated assuming they start at the t0 of the simulation. This means we will only use the 'correct' frequency plan if the simulation start time coincides with the start time of the orbits used to compute the frequency plan.

We should instead interpolate using the t0 saved in the frequency plan file. Note that this is currently not possible since the frequency planning files are defined with a different time reference. This has to be corrected before solving this issue.