Inaccurate butter4 bandwidth filter

[Wuhyun]

The current butter4 bandwidth filter obtained through tod_ops.filters.get_bpf() is inaccurate as it defines bandwidth filter as a product of a lowpass and a highpass filter. The resulting filter is not maximally flat and often yields a sub-1 frequency response at the bandpass centre. Instead, a Butterworth bandwidth filter should be calculated directly.

Demonstration using scipy:

Example application to white noise:

Example application to bandpass filtering needed for demodulation:

Note that the sine2 filter doesn't have this issue unless trans_width >= bandpass_width.

[Wuhyun]

Created #1143 to address this issue.