Add option to remove junk radiation before evaluating polarizations

[vaishakp]

The current behavior in WaveformModes.get_td_waveform does not allow for removal of junk radiation from the data.

This PR achieves the following:

1. Removes Junk portion from WaveformModes obj before evaluating polarizations
2. Allows for a choice of Junk time.

Desirable:

1. Fetch Junk radiation time from WaveformModes obj

[vaishakp]

Tests on GT catalog failing as the reference-time is not the key used in their metadata.

Need to implement retrieving junk time with a uniform attr across catalogs

[prayush]

For catalogs that do not provide a reference-time by this or any other name, in simulation/catalog metadata, are those that provide the waveform after removing junk portion of it. Therefore for those catalogs, I suggest initializing reference-time=0 by default for all simulations, unless specifically provided an overriding value.

[prayush]

Please cherry-pick #51