  channelShiftVec returns all nans if input spectra contains nans.

I have regions of nan values at the beginning and end of my cube due to regridding. When the input spectrum contains any nans, the channel shift returns all nans.

https://github.com/low-sky/spectral_stack/blob/c74e5b06d4385a5f8cc6d386f6c93334234c2510/stacking.py#L6

