Add `source` kwarg to `Workspace.get_gate_events`

[whitews]

Add source kwarg to Workspace.get_gate_events to retrieve gated events in various stages of preprocessing (original, compensated, transformed). This option would be consistent with Sample methods. Also, this is made possible by the requirement in FlowJo for a compensation and single set of transforms for each channel (unlike in GatingML / Session where that is not guaranteed).