Phase I and Branch and Bound

Phase I of the simplex method for finding an initial basic feasible solution
has been implemented. If an initial basic feasible solution is not provided to
the simplex function, Phase I is used to generate one.

Branch and bound has now been implemented as well. Given an LP in either
standard equality or inequality form, branch and found finds the optimal
all integral solution. No visualization has been implemented yet.

Previously, the LP class limited the creation of LPs to those in standard
inequality form. Although only standard inequality form LPs (in 2 and 3
variables) can be visualized, LPs can be instantiated in either standard
equality or inequality form. Furthermore, the b vector in Ax <= b or Ax == b is
no longer restricted to be nonnegative.