Part1: Radix-2 FFT enhancement by working on two layers per iteration

[lamphamsy]

The base branch eh/fft_radix2_and_re_work_sim would be merged into master when two parts are done: #253 #254

Part1

• Butterly CT on two-layers at a time

For each quadruple (P, Q, R, S) = (buf[i], buf[i + m], buf[i + 2 * m], buf[i + 3 * m])

First layer: butterfly on (P, Q) and(R, S) for step = 2 * m

      coef r1 = W[start * n / (2 * m)]
      P = P + r1 * Q
      Q = P - r1 * Q
      R = R + r1 * S
      S = R - r1 * S

Second layer: butterfly on (P, R) and (Q, S) for step = 4 * m

      coef r2 = W[start * n / (4 * m)]
      coef r3 = W[(start + m) * n / (4 * m)]
      P = P + r2 * R
      R = P - r2 * R
      Q = Q + r3 * S
      S = Q - r3 * S

• Remove useless butterfly operations on GF

[slaperche-scality]

Your explanations are nice, IMHO you should also put them in the message of the relevant commit.

That way they can be accessible directly from Git log (no dependence to github, can dive into the code offline, …).

[lamphamsy]

@slaperche-scality : Comments are addressed. Could you review the PR? Thanks