tBuLi/symfit: symfit 0.5.0

The long awaited symfit 0.5.0 is here. And it was worth the wait, because it has got some very significant improvements over previous versions, including:

• Hessian minimizer trust-constr added, and all our models and objectives support hessian minimizers fully.
• The components of models can now depend on each other, which greatly improves readability, allows for more creative definition of models, and means that expensive to calculate intermediates can be reused by other components.
• MatrixSymbols are now supported, meaning a vast number of Matrix equations can now also be fitted with great ease.
• Improved printing of fit results.
• Better checking if data is provided for all variables.
• BivariateGaussian added as a predefined distribution.
• A whole sway of minor and mayor bug fixes.