"Threshold" gating from the Excel UI

[castillohair]

Once #246 is implemented, we should have a gate called "Threshold", which would keep events below or above a specified value.

One thing to debate is whether one should be able to specify both a lower and an upper value, or just a single value and then choose whether to keep events below or above. I would favor the first option, which would make this gate functionally identical to high_low.
Another issue is that if we want to map the gating method to be specified in the Excel UI with a function name from the gate module, we would need a new gating function called threshold(). But this would be redundant with high_low. Should we use the name High-Low in the Excel UI? or should we change the name of the current high_low function in the gate module? I would favor the latter, as I think the name high_low is pretty uninformative.