Preprocess: Combine detector time constant and readout filter deconvolution

[msilvafe]

The filters module is made to be chainable so we can multiply the readout iir filter by the detector time constant filter then just apply one combined filter to the data. This is more efficient because it takes fewer fft/rffts. The preprocess fft_filter method needs to be updated to allow chainable filters or a specific method should be written to combine these two specific filters.