Branch and Bound Visualization

The visualization for the branch and bound algorithm (bnb_visual) is now
complete. Like other visualization functions, it takes a standard inequality LP
in either 2 or 3 variables. It then visualizes branch and bound assuming all
variables must be integral. On the left subplot, the remaining feasible region
that has not been eliminated from some bound is shown as well as a feint
outline of the initial LP feasible region. The current node's feasible region
is darkend and the simplex path is shown on it. On the right subplot, the
branch and bound tree is shown with the bound, objective value, and solution
for each node. Explored nodes are colored in and the current node is darkened
like it's feasible region on the left subplot.

In addition to this added functionality, there were numerous bug fixes, code
restructuring, and cosmetic changes.