qtgui: fixed frequency scale for float frequency plots.

Was setting bandwidth to half what it should have. Tested QWT 5.2.0 - 6.1.0.
filter: Python pfb.arb_resampler calculating better taps when rate < 1.0.

We have to change the concept of the channel bandwidth out of the resampler when the rate is < 1.0. Previously, we were getting aliasing by using the wrong bandwidth concept.