add option to specify open/closed overlap testing

[kip-hart]

Added the option to specify open vs closed overlap testing, per request in (#13). Default behavior remains unchanged if the closed flag is not set.

If closed is set to false, there must be a non-zero amount of overlap between boxes for there to be an intersection. This is the default behavior.

If closed is set to true, two boxes can be touching for there to be an intersection.

For example:

from aabbtree import AABB
from aabbtree import AABBTree
tree = AABBTree()
aabb1 = AABB([(0, 0)])
aabb2 = AABB([(-1, 0)])
tree.add(aabb1, 'box 1')
tree.add(aabb2, 'box 2')
# Open Boxes
v = tree.overlap_values(aabb2)
print(v)
>>> ['box 2']
# Closed Boxes
v = tree.overlap_values(aabb2, closed=True)
print(v)
>>> ['box 1', 'box 2']