prob_log about inferpy

[rcabanasdepaz]

Now log_prob and sample methods use dictionaries where the keys are strings with the variable names:

>>> model.sample()
{'y': array([[[4.87281385, 6.11311355]]]), 'x': array([[[ 0.80738106,  0.68161732],
        [-1.19498769,  0.93624021],
        [-0.2377974 ,  0.05938201]]])}