FFTs and optimisation

Changing the FFT from FFTW to av_fft made little or no difference
on the CPU load when running DVB-T. However optimising the
RS encoder has taken a few percent off the CPU load.

I am using the Valgrind tools to profile the program, the highest CPU load
varies between the iFFT the RS encoder and the interleaver. Currently
the interleaver is the biggest CPU drain and I can't find any obvious way
to optimise it.

I have been able to get 2 MHz wide DVB-T to work on the Odroid U3+
but I had to reduce the oversampling which has caused a small amount
of aliasing to appear in the output spectrum. Work continues.