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Parks-McClellan FIR filter design. 
 
Calculates the optimal (in the Chebyshev/minimax sense) FIR filter inpulse reponse given a set of band edges, the desired reponse on those bands, and the weight given to the error in those bands. 
 Frequency is in the range [0, 1], with 1 being the Nyquist frequency (Fs/2)Parameters:
  
    | order | filter order (number of taps in the returned filter - 1) |  | bands | frequency at the band edges [ b1 e1 b2 e2 b3 e3 ...] |  | ampl | desired amplitude at the band edges [ a(b1) a(e1) a(b2) a(e2) ...] |  | error_weight | weighting applied to each band (usually 1) |  | filter_type | one of "bandpass", "hilbert" or "differentiator" |  | grid_density | determines how accurately the filter will be constructed. \ The minimum value is 16; higher values are slower to compute. | 
 
 Returns:vector of computed taps
 Exceptions:
  
    | std::runtime_error | if args are invalid or calculation fails to converge. | 
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