This function represents a simple weighting function for spectral filtering.
BP(f, fc, BW, n = 3)vector of frequencies
center frequency
bandwidth, with w[f < (fc - BW) | f > (fc+BW)] == 0
degree of the polynom, n can be real, e.g. n = 2.5
This function returns a weight vector, which is to apply to the frequency
vector f in a top level function
The band pass is represented troughout a polynom in the form
$$w = 1 - a * (f-fc)^n$$ with the degree \(n\). The parameter \(fc\)
controlls the center frequency and \(a\) scales the required band width BW.
outside the band width the result is forced to zero.