Test spectral notch with ofdm-notch-test branch

[jgaeddert]

There are essentially two (untested) changes in this branch:

1. Increased the number of subcarriers from 32 to a more reasonable 200
2. Added a notch of 25 subcarriers (12.5% bandwidth) offset from DC

With this change, the data should still go through fine, but the band should be visibly non-contiguous. Furthermore, an interferer placed within that notch should have little to no effect on performance.