Plot.get / renderGates requires `renderGates` to be a string instead of bool

[gingo00]

I had some difficulty getting CellEngine plots with the gates rendered, even if I used the right x/y channels, parent population id and compensation id along with the "renderGates" parameter set to True. The plots were always generated without the gates drawn on them. I finally figured out that I had to pass the "renderGates" parameter as a string ("true") for it to work.

The documentation describes this parameter as a bool, but for some reason, using python booleans do not work as one would expect.