[FEA] FFT-based DDC
Currently, a Polyphase resampler is used as the base of the DDC. The polyphase resampler allows rational downsampling and is more flexible than an FFT-based downsampler. However, when an integer downsampling is required, an FFT-based can be significantly faster and provide more independent DDC bands and many frequency-continuous DDC bands. The DDC processor should auto-select between both approaches if it is not explicitly set.