[CM] make-bandpass xcoeffs

[bil at ccrma.Stanford.EDU]

make-bandpass is a wrapper around make-fir-filter, returning
an fir-filter generator, so mus-xcoeffs does work with it;
there's an example in snd-test.scm around line 7706.
make-bandpass is not itself a generator; it returns one.
If you look at the code in dsp.scm you'll see
(define bandpass fir-filter) -- this says the
bandpass generator is an fir-filter.  In the
first case, I think you're setting a vector to the
output of the bandpass generator, and in the second
it looks like you've copied the make-bandpass code
to set the vector, so you're setting the vector to
two different things.