Occasional duplicate frequencies on Q-planes

Created by: alurban

Occasionally, when computing Q-transforms of data streams down to very low frequency (~0.01 Hz), the floor division in QPlane._iter_frequencies results in duplicate frequencies on some fixed-Q planes. A fix for this is on its way imminently.

This issue was originally reported by @dethodav after running gwdetchar-omega with the following configuration:

[PEM]
name = Physical Environment Monitoring
q-range = 3.3166,50.0
frequency-range = 0.03,inf
frametype = L1_R
duration = 512
fftlength = 8
max-mismatch = 0.35
snr-threshold = 5.5
always-plot = True
plot-time-durations = 1,4,16
channels = L1:PEM-EX_SEIS_VEA_FLOOR_QUAD_SUM_DQ

Since this PEM channel is low-sample-rate, the very low frequency behavior is meaningful, and needs to be analyzable by omega scans.