Add BoundingBox union and intersection

[larrybradley]

Examples:

>>> from regions import BoundingBox

>>> bbox1 = BoundingBox(1, 10, 2, 20)
>>> bbox2 = BoundingBox(5, 21, 7, 32)

>>> bbox1.union(bbox2)
BoundingBox(ixmin=1, ixmax=21, iymin=2, iymax=32)

>>> bbox1 | bbox2
BoundingBox(ixmin=1, ixmax=21, iymin=2, iymax=32)

>>> bbox1.intersection(bbox2)
BoundingBox(ixmin=5, ixmax=10, iymin=7, iymax=20)

>>> bbox1 & bbox2
BoundingBox(ixmin=5, ixmax=10, iymin=7, iymax=20)

[cdeil]

Thank you!