Release Version 0.2.1

• Add the tutorial for Gaussian process regression.
• Fix empty operator bug.
• Fix bug to allow the same variable for multiple inputs to a factor.
• Add module serialization.
• Fix the bug that causes the failure of documentation compilation.
• Fix the bug: the inference methods of GP modules do not handle samples.
• Update issue templates.
• Add license headers to all files.
• Add the getting started notebook.
• Remove the dependency on future.
• Update the Inference documentation.
• Implement Dirichlet distribution.
• Add logistic variable transformation.
• Implement Expectation inference.
• Fix the bugs related to dtype.
• Validate shape of array variables.
• Fix divide by zero error if max_iter < n_prints.
• Add multiply kernel.