Example in LinearGaussianBayesianNetwork.add_cpds doesn't work

[chaonan99]

Subject of the issue

Example code in the docstring of LinearGaussianBayesianNetwork.add_cpds doesn't work.

Your environment

• pgmpy version: 0.1.6
• Python version: 3.6
• Operating System: Ubuntu 14.04

Steps to reproduce

    Examples
    --------
    >>> from pgmpy.models import LinearGaussianBayesianNetwork
    >>> from pgmpy.factors.continuous import LinearGaussianCPD
    >>> model = LinearGaussianBayesianNetwork([('x1', 'x2'), ('x2', 'x3')])
    >>> cpd1 = LinearGaussianCPD('x1', [1], 4)
    >>> cpd2 = LinearGaussianCPD('x2', [-5, 0.5], 4, ['x1'])
    >>> cpd3 = LinearGaussianCPD('x3', [4, -1], 3, ['x2'])
    >>> model.add_cpds(cpd1, cpd2, cpd3)
    >>> for cpd in model.cpds:
            print(cpd)

This cause error AttributeError: 'LinearGaussianCPD' object has no attribute 'variables', which is obviously caused by missing attribute in LinearGaussianCPD. A simple solution is to add a property

@property
def variables(self):
    return [*self.evidence, self.variable]

[chaonan99]

Actually the same issue with #961