Inference-tools

- GpRegressor now supports multi-start gradient-based hyper-parameter optimisation using the [L-BFGS-B algorithm](https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.fmin_l_bfgs_b.html), in addition to [differential evolution](https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.differential_evolution.html#scipy.optimize.differential_evolution), which was previously available. Which of these approaches is used can be selected using the new "optimizer" keyword argument, with L-BFGS-B being the default.

- GpRegressor now supports distributed hyper-parameter optimisation using sub-process based parallelism. The number of sub-processes over which the optimisation is distributed is set by the new "n_processes" keyword argument. Currently only the multi-start L-BFGS-B optimiser can take advantage of this, so this keyword is ignored when using the differential evolution optimizer.

- Fixed a bug in GaussianKDE which caused a crash when fewer than 10 samples were given as input.

- Rather than assuming the mean of the Gaussian process is zero, `GpRegressor` now treats the mean as a hyper-parameter, and automatically selects a value for the mean which best describes the data.

- Fixed a bug in the calculation of the derivatives of the log-marginal-likelihood and the log-cross-validation density with respect to the hyper-parameters.

- Added new set of Jupyter notebook demos, which can be found in the `/demos/` directory

- Added a new function `inference.plotting.hdi_plot` for convenient plotting of highest-density intervals derived from a sample of model realisations.

- All sampling classes in `inference.mcmc` now pass model parameters to the user-provided posterior function as a `numpy.ndarray`, and the documentation has been updated to reflect this.

- Fixed a bug introduced in the 0.5.0 release where a passing single spatial point to the `__call__` method of `GpRegressor` would cause a crash in cases with 2 or more spatial dimensions.

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 reduced computation times.

- Fixed various bugs that appeared when testing after updating dependencies to numpy 1.15.0, scipy 1.3.1 and matplotlib 3.1.1