Egobox

* Fix GP mixture with kpls option on Griewank test function by relf in <https://github.com/relf/egobox/pull/150>

* [Breaking changes] `gp`, `moe`, `egobox` (Python): Rename `predict_derivatives()` as `predict_gradients()` by relf in <https://github.com/relf/egobox/pull/148>

* [Breaking changes] `gp` API renaming by relf in <https://github.com/relf/egobox/pull/145>
* `predict_values()` is renamed `predict()`
* `predict_variances()` is renamed `predict_var()`
* `predict_variance_derivatives()` is renamed `predict_var_derivatives()`
Derivatives predictions (`predict_derivatives()` and `predict_var_derivatives()`) are made available in Python.
* Refactor Mixture of Experts by relf in <https://github.com/relf/egobox/pull/146>
Factorize code between full GP and sparse GP implementations

* Add `Gpx` accessors by relf in <https://github.com/relf/egobox/pull/140>
* Fix `LHS` maximin bug by relf in <https://github.com/relf/egobox/pull/141>
* `doe`: Improve classic, centered and maximin LHS performances by relf in <https://github.com/relf/egobox/pull/138>
* `doe`: Improve optimized LHS performances (1.25x speedup) by relf in <https://github.com/relf/egobox/pull/136>
* Rework (mostly internal) API to avoid awkward &Option by relf in <https://github.com/relf/egobox/pull/134>
* Add Python bindings for all LHS flavours by relf in <https://github.com/relf/egobox/pull/135>

* `gp`: Implement sparse gaussian process methods (cf. `SparseGaussianProcess`)
* Python binding: `SparseGpMix`, see doc/tutorial
* GP/SGP API
* hyperparameter tuning : initial theta guess and bounds can be specified (`theta_init`, `theta_bounds`)
* `n_start` controls the number of optimization multistart
* In GP/SGP `rayon` is used to make parallel optimization multistart

* `ego`: Fix ask-and-tell interface `suggest()` method in presence of discrete variable to work
in discrete not in continuous space
A few API breaking changes:
* `EgorConfig::xtypes` not an option anymore
* `EgorSolver::new_with_xtypes()` renamed `new` as `new` with xlimits is removed, use `to_xtypes` to convert `xlimits`
* `EgorConfig::no_discrete` attribute removed, use `EgorConfig::discrete()` method
* `SurrogateBuilder::new_with_xtypes_rng` renamed `new_with_xtypes`