StandardApplet: Allow subclasses to provide init args for their single-lane top-level operators.
FeatureSelectionApplet: Filter implementation (original, refactored, or interpolated) is passed in as a flag to __init__
OpFillMissingSlices: "Regular" output does not use a cache, but there is now an CachedOutput for the GUI to use.
FeatureSelection: Fixed calls to OpFeatureSelection constructor to match new __init__ signature.
Refactored OpThresholdTwoLevels, added ThresholdTwoLevelsApplet, and added two special-purpose workflows for the two parts of the synapse detection pipeline.