having an executable script in a Pre Condition

[ufrhaidar]

Similar functionality to _post.

It is good for many things as it can help the tree be more readable:

• resetting or making sure a variable exists
• updating status to know a node has started.

[facontidavide]

yes, I have been thinking about that. I think that I will add it

[Aglargil]

yes, I have been thinking about that. I think that I will add it

@facontidavide This PR might help solve this case. Let me know if you need any clarification on the implementation.
#899