Modulo-n circular convolution

`c = cconv(a,b)`

`c = cconv(a,b,n)`

For long sequences, circular convolution can be faster than linear convolution.

[1] Orfanidis, Sophocles J.
*Introduction to Signal Processing*. Englewood Cliffs, NJ:
Prentice-Hall, 1996, pp. 524–529.