Preprocess: Need to restrict noise fitting in low passed data

[msilvafe]

Currently the demodulation lowpass filter is set by the hwp rotation speed if the filter parameters are not specifically provided. The noise fitting function should provide a check that frequencies above the lowpass filter are not included in the fits and raise a warning. @kmharrington raised this because she was analyzing stare observations where the HWP rotation speed was varying in each obs. The lpf was adjusting but the white noise/noise fit region was not fit accordingly so the results were biased leading to an erroneous interpretation of a large dependence of our noise levels on our choice of HWP rotation speed.