replace custom fft, ifft functions with built-in functionality

[fietew]

Things are quite confusing with fft_real and ifft_real, as they do not provide any additional functionality which is not provided by MATLAB's fft and ifft. Furthermore, convolution can be improved, if x is a vector and y is a matrix (or vice versa), as the FFT has to be only performed once on the vector.

[VeraE]

I tested the new convolution() function and it works nicely. To me, the code is now more obvious, too.