Fluorescence gating from the Excel UI

[castillohair]

Same idea as #246 and #269 but with fluorescence channels.

Two unique complications for this feature are

1. How to specify which fluorescence channels to use for each gate. Probably specifying channels as additional parameters would work fine.
2. How to draw the gates, especially when performing 2D gating. This is a complication because the Excel UI currently only draws 1D histograms of each sample.