Import QucsStudio Filter Synthesis source code

Import QucsStudio Filter Synthesis source code

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Draft change log:

implementations
  lc_filter.cpp    // ladder filter containing inductors L and capacitors C
  qf_poly.cpp      // Class for polynoms with real coefficients (R[X])
  qf_filter.cpp    // Generic filter class
  qf_cauer.cpp     // Elliptic (Cauer) filters, odd order
  cline_filter.cpp // coupled transmission line filter
  eqn_filter.cpp   // equaton defined filter
  line_filter.cpp  // capacitive end-coupled, half-wavelength bandpass filter
  stepz_filter.cpp // stepped-impedance transmission line filter
  tl_filter.cpp    // transmission line filter

class
  lowpass
  highpass
  bandpass
  bandstop

type
  bessel
  butterworth
  chebyshev
  cauer