breeze-718: remove hard coded "maximize" in LinearProgram.Problem.toString

[qxzzxq]

Description

This PR should close #718

• A method goal was added into the trait Problem.

  sealed trait Problem {
  outer =>
  def goal: Option[GoalType] = None
  ...
  }

• Two methods (maximize and minimize) were added into the class LinearProgram

  def minimize(expression: Expression): Problem
  def maximize(expression: Expression): Problem

  The two new methods will override the method goal during the instantiation of a Problem object. When we call Problem.toString, the output of goal will be used instead of the hard coded "maximize"

• A check was added to prevent from calling maximize() on a minimization problem

• A method solve was implemented in the trait Problem. Il will invoke the corresponding method according to the value of goal

Example

val lp = new LinearProgram()
import lp._

val x0 = Real()
val x1 = Real()

val min = minimize(x0 + x1)
  .subjectTo(x0 >= 20)
  .subjectTo(x1 >= 30)

println(min.toString)
// minimize    x_0 + x_1
// subject to  x_0 >= 20.0
//             x_1 >= 30.0

min.solve  // this equals to minimize(min)

maximize(min)  // this will throw an exception