Frequency vectors: a correct way

These are (auxiliary) functions for computing (correct) 1 or 2-sided frequency vectors. These are not being exported (at least yet) but could be used internally to get frequencies. Compare:
```julia
# this 2-sided
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,-7.0,-6.0,-5.0,-4.0,-3.0,-2.0,-1.0]
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0,-7.0,-6.0,-5.0,-4.0,-3.0,-2.0,-1.0]
# currently from spectrogram (after converting to vector)
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0,11.0,12.0,13.0,14.0,15.0]
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0,11.0,12.0,13.0,14.0]
# this 1-sided
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0]
[0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0]
```