Obb SAH

This PR is the first in a series of PRs intended to increase the quality of OBBTrees generated in MOAB.

This contains several changes to the OrientedBoxTreeTool and OrientedBox classes including:

• a refactor of the build method into generic tree generation methods like link_node, split_node, create_node, and create_leaf_node

• extention of the OrientedBoxTreeTool::Settings structure to allow for multiple split cost heuristics

• the addition of a simplified Surface Area Heuristic (SAH) for evaluating box split costs (more details in the documentation of the new method)

It is worth nothing that the default behavior (heuristic and settings) of the OBBTree is the same after these changes, but that it now allows for different options to be accessed if desired by an exterior application (in our case, DAGMC)

For reviewers I'd like to make a couple things clear. I'm:

a) open to any critique as to the implementation and/or design of these new features in the tool

b) not in a large hurry to have this merged and am hoping for some discussion surrounding the changes

basically, this is going to contribute to my thesis work so I'd these PRs to be a lasting, valuable contribution to MOAB and as such I want to get it right :)