efficient overlap-save for real signals

Performing overlap-save method for convolution of a long sequence with an impulse response.

overlap–save is an efficient way to evaluate the convolution between a very long signal x[n] and a filter h[n]. In the case where the signals are real we can compute the output even more effiecently. In this fucntion we implement overlap-save for real signals i.e it is sufficient to keep only the positive frequencies of the fft and when computing the inverse fft we can simply complete the negetive frequencies (for real signals notice conjugate complex symmetry).