C-bowman/inference-tools: 0.5.0 release

This release contains significant improvements to the GpRegressor class, including:

• A new option to select between the squared-exponential and rational-quadratic covariance functions, or provide a user-defined custom covariance function.

• A new option to use leave-one-out cross-validation to select hyper-parameter values instead of the marginal-likelihood.

• Significant improvements to numerical efficiency leading to reduce computation times.