Add conversion from Expression to Objective

[tlambert03]

This extends Expressions added in #9 to convert to Objective objects (with an optional sense argument).

Note @funkey : this also changes the name of the method constraint() to as_constraint() ... (and as_objective here). I think it reads a bit clearer, you?

from ilpy import Sense
from ilpy.expressions import Variable

u = Variable("u", index=0)
v = Variable("v", index=1)
e = Variable("e", index=2)

expr = 2 * u - 5 * v + e / 2 - 3

objective = expr.as_objective(Sense.Maximize)  # optional arg ... default is Minimize
assert objective.get_constant() == -3
assert objective.get_sense() == Sense.Maximize
assert objective.get_coefficients() == [2.0, -5.0, 0.5]

[tlambert03]

@funkey ... i'll take care of the cleanup here. you can just check if the general pattern looks ok

[funkey]

...too late ;)

[funkey]

Haha, I see, I missed a few more. Sorry for the noise, and thanks for cleaning up :)