b = firls(N, F, A);
b = firls(N, F, A, W);

FIR filter design using least squares method. Returns a length N+1
linear phase filter such that the integral of the weighted mean
squared error in the specified bands is minimized.

F specifies the frequencies of the band edges, normalized so that
half the sample frequency is equal to 1.  Each band is specified by
two frequencies, to the vector must have an even length.

A specifies the amplitude of the desired response at each band edge.

W is an optional weighting function that contains one value for each
band that weights the mean squared error in that band. A must be the
same length as F, and W must be half the length of F.