Multiple scatter gates specified from the Excel UI

[castillohair]

Pending acceptance of #246. See pull request for more details on discussion.

The idea is that multiple gates can be specified as a comma-separated list in the column "Scatter Gate" and gating parameters in individual columns. FlowCal should then process these sequentially.

An open issue is how to represent the application of multiple gates in the FSC/SSC plot. From #246, the current ideas are:

• Draw the contour of each gate in black, as it is done now for the single density gate that we have. I think this can get pretty confusing when you have multiple gates.
• As above, but draw events that are gated out with a higher degree of transparency. In this way, it is obvious which region is the final gated region.
• Draw each individual contour in grey, and a contour that only delineates the final selected population with black. How to calculate the final contour is something I don't know.

Some testing of these different representations would be useful. We might also want to wait until we have more gates (e.g. #268) to implement and test this.