Errors (IndexError) on silent waves (degenerate pathological cases)

```python
File "/home/nyanpasu64/miniconda3/envs/wavetable/lib/python3.6/site-packages/waveform_analysis/freq_estimation.py", line 92, in freq_from_autocorr
start = find(d > 0)[0]
IndexError: index 0 is out of bounds for axis 0 with size 0
```

If `freq_from_autocorr` is given a silent signal `d` contains a series of 0.

I feel the best solution is to just return some arbitrary value without crashing.

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I don't remember if it happens in practice, but a while back I found that `parabolic(f, 0)` compares to f[-1] (which is wrong but doesn't crash) and `parabolic(f, len(f)-1)` compares to f[len(x)] which raises IndexError.