Instead of having two ways of using abilities directly, have one
way which is consistent with the API. Now unit has one method called
`ability` which takes a AbilityID and optionally either a Position or
another Unit which to use the ability at.

fixed indents to match previously written code

returns true if unit has target

simple python edit

Integrate all information from TechTreeImproved into the existing class
TechTree. Thus TechTreeImproved is deprecated and removed from the
Python-API.