Task Update

[Ryugoron]

Implement a task update rather than a task remove on collision of tasks.

Example

Manipulating a side condition can kill tasks of an agent. For example a clause 'c' considered for splitting will be canceled if another one was executed first.

With the collision removal the new split states have now to be observed for any existing formula, if a split can performed over this one, instead of just reassign the splitting node of the already computed ones.

This has a high relevance in the example tableaux implementation, since alpha rules are deleted every time and unnecessary operations may result.