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.

Feature branch allowing for using abilities directly and viewing current buffs on units.

Feature branch allowing to set the number of attempts when looking for a
valid build location. Setting the number to 0 will do an exhaustive
search.

Closes merge request !4.

Changed types. get_build_location_near accepts a non-negative integer: 0 indicates an exhaustive search

Added methods to use abilities.

See merge request !3