Update jointplot to handle competing risks

[graemeleehickey]

Update jointplot to handle competing risks

[graemeleehickey]

The setup of this code makes this task non-trivial. Currently, the code groups all non-censoring events (therefore pools all failure events) into a single failure group and plots as normal. For now, this seems reasonable, and a warning + documentation note have been added. In future, it would be useful to replace all these plotting function.