Revise Task interface

[azawlocki]

• We have both computation_finished() and finished_computation() methods
• query_extra_data() should probably be changed to a better name
• Is there a difference between needs_computation() and get_tasks_left() != 0?

[badb]

Remember to check wiki

1. finished_computatation is a question, computation_finished information. Do you have better ideas for names?
2. Maybe get_next_subtask?
3. In current tasks there is no difference. We can imagine some scenarios where it may be a difference, but in most cases it should mean the same.

[azawlocki]

1. maybe on_computation_finished, since this is kind of a callback? Or react_to_computation_finished. Some general convention would be useful here.
2. sounds good
3. I see

[badb]

1. Personally I prefer react_to_computation_finished (convention with verb), but other proposition is also good.