Density gating should be optional in the Excel UI

[castillohair]

I think an additional column called "Scatter Gating" should be added, in which the option "Density" would result in the density gate that we currently use. "None" would result in no FSC/SSC gating. Future methods could include "Gaussian", similar to what TASBE does.

If the column "Scatter gating" is not present, it would be cleaner to not use any gating. But to preserve backwards compatibility we could default to density gating. I'm fine either way.