Started a work on the circular wall generator.
[Does not works currently.]
Wall generator almost done

The wall generated is round excepted a small part at the middle of a quater of the circle.
Fixed the circular wall generator. It works!

Also:
 - improved the squared wall generator (no more "solidBlock" towers in the corners).
Fixed a bug about the wall checker in circular world.

The players on an angle of a circular wall were considered as out the wall by WorldBorder/the custom checker, because the check is performed against a real circle and not a pixelized one.
Merge branch 'circulargen' of github.com:AmauryCarrade/UHPlugin