Unit-length waveform period (alternating signs for gradients)

[artfwo]

Using alternating signs for even & odd gradients in the table could solve 2 problems:

1. Cusps and distortion when the outermost gradients are either both positive or both negative.
2. We can use only 2 gradients instead of 3 to create one whole symmetric oscillation. To keep the resulting sound loud, normalization (issue Output level normalization #3) should be implemented as well.