Remove tunnelling

[Yonaba]

This feature is not likely to be useful (included in 1.8.1). It sounded like something good,at first, but it should be removed in the next release. Plus, it does not well acommodates with most of search algorithms, such as Jump Point Search, ThetAstar and Lazy ThetAstar.
The feature will be removed, in favor of allowCornerCrossing (see #30)