Copulae 0.6.0

This release brings in the following changes

1. The addition of the Gaussian Mixture Copula Model (GMCM).
2. Backport the code to support Python 3.6
3. A modest refactoring of the copula's Estimator class
4. A modest refactoring to combine the AbstractCopula and BaseCopula class while retaining type hintings (for PyCharm)