Add possibility to specify actions for output variables

[tudortimi-ifx]

Currently, only return values of functions can be specified. It's also possible to use output arguments to transfer information from a function to its caller. While this may be considered bad practice for functions, it's the only way to implement "non-void" tasks (i.e. tasks that "return" something via an output argument).

It should be possible to specify the values output arguments take when tasks/functions finish.

[nosnhojn]

this is on my list. tad bit messier than the the returns() though so it's lower priority until we get a bit more feedback/usage of what's there