Visualize update

[roussel-ryan]

• updates default plotting for acquisition function, we now plot Exp[\alpha] for more intuitive visualization
• also reduces computational load of plotting in the mobo examples to encourage documentation action passing

[ChristopherMayes]

Unless the numerical value of $e^\alpha$ has some meaning, it will be better to use a logarithmic color map in matplotlib:

from matplotlib.colors import LogNorm
plt.imshow(data, norm=LogNorm())

[ChristopherMayes]

This needs a better docstring:

help(X.generator.visualize_model)

Help on method visualize_model in module xopt.generators.bayesian.bayesian_generator:

visualize_model(**kwargs) method of xopt.generators.bayesian.mobo.MOBOGenerator instance
    displays the GP models