Add space between operators

[gerazov]

This could be related to #28

The math is:

  y[n, i] = x[n, i] - \frac{1}{N} \sum_{n=0}^{N-1} x[n, i] \quad i = 0, 1 \, ,

And the rendering:

Which looks great, but it would be nice to have space around the fraction.