Wait this PR is about HFFT, not DCT.  I'm confused.

Anyway, the point is to show how hfft produces half the output of fft for the same input.  irfft would do the opposite:

```
>>> from numpy.fft import irfft
>>> spectrum = np.array([ 15, -4, 0, -1, 0, -4]) 
>>> irfft(spectrum[:4])
array([ 1.,  2.,  3.,  4.,  3.,  2.])
>>> ihfft(spectrum).real
array([ 1.,  2.,  3.,  4.])
```