Create a conditional task

[haxsaw]

We need a task that wraps one or two other tasks and a test of some kind. If the test returns True, then one task is run. If false, either nothing happens or the other task is run. Something like this:

CondTask(name, predicate, true_task, else_task=None)

where:

• name is the conditional task name
• predicate is a callable that takes a CallContext and returns either True or False
• true_task is some kind of task that is run if predicate returns true
• else_task defaults to none, but can be task that will be run if predicate is false

CondTask only fails if one of its wrapped tasks fails.