added option for unique/non-unique returns

[kip-hart]

Addresses the first comment in (#15) for returning duplicated AABBs in the tree. The default behavior is preserved. To return duplicates, use the option unique=False.

[kip-hart]

expected behavior tested in:

AABBTree/tests/test_aabbtree.py

Lines 265 to 278 in 11eb24b

	def test_unique():
	tree = AABBTree()
	aabb1 = AABB([(0, 1)])
	aabb2 = AABB([(0, 1)])
	aabb3 = AABB([(0, 1)])
	tree.add(aabb1, 'box 1')
	tree.add(aabb2, 'box 2')
	vals = tree.overlap_values(aabb3, unique=True)
	assert len(vals) == 1
	vals = tree.overlap_values(aabb3, unique=False)
	assert len(vals) == 2
	assert 'box 1' in vals
	assert 'box 2' in vals