Directly support tasks with multiple endpoints

[justin13601]

• Enable OR relationship across branches in the task tree

[justin13601]

It is currently tricky to express tasks that have multiple endpoints. For instance, to extract a cohort for 30-day mortality, you would require patients that died within 30 days of a given timestamp (ie., contains death predicate in a window), as well as patients that did not die within the same 30 days (ie., patients with data 30-days past the given timestamp). An overlapping 30-day window could be used to express this, which would create two branches in the task tree. However, it is not yet possible to express an OR condition across branches (ie., the AND relationship is evaluated across different branches). As such, to extract this cohort, you would also have to make use of two configuration files, one to extract a cohort of those that meet criteria for the first branch (ie., death within 30 days), and one for the other branch (ie., data past 30 days).