Optimization of sigma computation in OpenCV 2.4.8 SIFT implementation (Bug #3625)

We can see that, according to the original code :

<pre>
sig[0] = sigma
sig[i] = sqrt(sig_total * sig_total - sig_prev * sig_prev )
       = sqrt(sig_prev * sig_prev * k * k - sig_prev * sig_prev )
       = sig_prev * sqrt(  k * k -  1 )
       = k^(i-1) * sigma * sqrt(  k * k -  1 )
</pre>

then:
<pre>
sig[0] = sigma
sig[1] = sigma * sqrt(  k * k -  1 )
sig[2] = k * sigma * sqrt(  k * k -  1 ) = k* sig[1] 
sig[3] = k^2 * sigma * sqrt(  k * k -  1 ) = k* sig[2] 
</pre>

We can optimize as done in the PR.

I've also added clarifying comments because of a bug report that was incorrectly filed due to lack of commentary.

http://code.opencv.org/issues/3625