Fix time step setting in HeterodynedData object

When creating a new HeterodynedData object from data that was irregularly sampled, it subsequently sets the dt (time step) property of the data using the minimum time step between any of the supplied times. However, this has the unintended consequence of deleting the stored time stamps (with their irregular sampling) and then setting new time stamps based on the first time stamp and the newly created dt time step (see the dx setter for the GWPy Series and _update_index method that does this.

This MR instead sets a dt property of the HeterodynedData object, which is used instead and does not reset the time stamps.