Make backgrounds deterministic

• atomicAdd performs arithmetic operations in a nondeterministic order, and hence produces nondeterministic values when working with floating point types.
• Whitening uses FFTW configured with FFTW_MEASURE, which uses runtime performance to pick an FFT plan, and hence is not deterministic, so we instead use the deterministic FFTW_ESTIMATE
• postcoh_finalsink.py assumes os.listdir() to return files in lexicographic order, which is not guaranteed, so we sort it to make this assumption hold