created block mode only GUI
reference levels will only be applied if a reference level packet is received, if not, the previous reference level value will be used
add model 5k-220, -208 properties
added combo box to control zif/sh/hdr rfe modes in spectrum analyzer
add rfe_modes to device properties
revert some inadvertent changes
correct calculate_fft missing context workaround
Merge branch 'block_mode_speca' into mode-constants

Conflicts:
	pyrf/gui/device_widget.py
	pyrf/gui/gui.py
	pyrf/gui/gui_config.py
	pyrf/numpy_util.py
remove unneeded imports
remove not-yet-implemented rfe_mode parameter
fixes for mode constants
block mode: don't trim attenuated edge
capture device fix for rbw too large