Add Option to Remove Junk Radiation in WaveformModes

[anuj137]

It would be more efficient and convenient if there were an option to remove the junk portion of the radiation from the WaveformModes object before evaluating the modes. This would ensure that the final polarization modes, hp and hc, are free of spurious oscillations at the beginning. Below is an example of the default output of WaveformModes.get_td_waveform() for a particular SXS simulation, which shows the junk radiation at the start.

[vaishakp]

This will be handled by #51. Please close #49 afterwards.