#305 dau1997 (and maybe other models) fail when flow and fhigh are outside the range of basef

[Alejandro Osses]

Way to recreate the bug:
[insig,fs] = greasy;
[outsig,fc,mfc] = dau1997(insig,fs,'flow',200,'fhigh',200);

The problem is that in this case fc will be empty (because basef=1000 Hz by default) and therefore there is no fc that exactly matches flow=fhigh=200 Hz. This produces an empty outsig that fails when modfilterbank is run.