Drop DC Harmonic from stft

[kkappler]

Note that the STFT object may have inf/nan in the DC harmonic, introduced by recoloring in run_ts_to_stft.

We later get RuntimeWarning happening when we calibrate this term

  stft_obj[channel_id].data /= calibration_response```

Since we don't use the DC harmonic for anything, it maybe better to just drop that term from the STFT. 

[kkappler]

The top answer in this post could also be a solution, so we just suppress that particular warning. But with xarray we should be able to just restrict the stft_obj to frequencies strictly greater than zero.
We just need to be careful that band selection is being done with frequency, and not integer indices, otherwise this will cause a bias towards higher frequency in the results.