Added

• sos::SisoSosFilter, an implementation of cascaded second order sections
  • includes new_interpolated initializer, which interpolates the SOS coefficients versus cutoff ratio from a lookup table
  • generic over f32 and f64
  • FMA and non-FMA implementations, configured by the existing fma feature
• functions to generate SOS Butterworth lowpass filters from lookup tables: sos::{butter2, butter4, butter6}
  • provides orders 2, 4 and 6 for f32 and f64
  • cutoff_ratio region of validity is enforced and tested for each combination of filter order and float type
  • cutoff_ratio region of validity is larger than existing Butterworth implementation for orders 4 and up
    • e.g. for order 4, f32: old 0.032, new 0.005
  • SOS does not improve region of validity for order 2 filter
• python script to autogenerate the SOS Butterworth lookup tables: scripts/generate_butter_sos_tables.py
• benchmarking with criterion, and a benchmark of sos::butter4::<f64>

Changed

• !Renamed flaw::SisoIirFilter::initialize to set_steady_state