Generated noise TOD has wrong PSD

[tskisner]

It pains me to open this issue, but it looks like there is a problem with a few lines of code which results in generated noise timestreams having the incorrect PSD. The problem is that the Fourier domain vector that is input to scipy.fft.irfft expects complex values packed as pairs of real and imaginary values, and we have instead been passing in data in the half-complex fftw storage ordering. The result is that the PSD amplitude is applied in the Fourier domain to the wrong elements, resulting in some low frequency power being shifted to higher frequency. This failure mode actually still allows our unit tests (based on overall normalization) to pass.

This was discovered when looking at pull request #67 in more detail. That pull request fixes the issue as a side effect of switching the numpy fft package.

I'm working to fix this now, but wanted to open this issue for the sake of transparency.

[tskisner]

Here is a quick plot showing how the incorrect ordering of Fourier domain elements previously led to some power being shifted from low frequency to high.

[tskisner]

This is closed by bfd6b9b. The PSD estimated from the generated noise TOD now agrees with the input, and unit tests have been added to check this.