Fix LinearGaussianCPD member variables [fixes #961]

[theUltraMarine19]

This was causing a compilation error when declaring LinearGaussianCPDs and adding it to a LinearGaussianBayesianNetwork model

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Issue number(s) that this pull request fixes

• Fixes Variable naming inconsistency? #961

List of changes to the codebase in this pull request

• Replaced variables with self.variables in init method of LinearGaussianCPD.py
• Added a simple test test_cpds_simple which checks whether LinearGaussianCPDs have been initialized correctly and adds it to a LinearGaussianBayesianNetwork model to check whether it works.

[theUltraMarine19]

This is a PR which is a replica of #962

[ankurankan]

@theUltraMarine19 Thanks for the PR.