MultipointExpectedImprovement returns negated acquisition value and gradient

[BrunoKM]

Contrary to the interface of the other acquisitions, the evaluate and evaluate_with_gradients methods of MultipointExpectedImprovement acquisition return the negated acquisition value and gradient.

Example in evaluate:

emukit/emukit/bayesian_optimization/acquisitions/expected_improvement.py

Line 185 in 9f6763f

return -self._get_acquisition(mean.flatten(), variance, y_minimum)[0]

and in evaluate_with_gradients:

emukit/emukit/bayesian_optimization/acquisitions/expected_improvement.py

Line 201 in 9f6763f

return -qei, -qei_grad

Making an issue to summarize this aspect of the discussion in: #303 (comment)