Egobox

* [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`

* `ego`: API refactoring to enable `ask-and-tell` interface
* Configuration of Egor is factorize out in `EgorConfig`
* `EgorBuilder` gets a `configure` method to tune the configuration
* `EgorService` structure represent `Egor` when used as service
* Python `Egor` API changes:
* function under optimization is now given via `minimize(fun, max_iters-...)` method
* new method `suggest(xdoe, ydoe)` allows to ask for x suggestion and tell current function evaluations
* new method `get_result(xdoe, ydoe)` to get the best evaluation (ie minimum) from given ones

* `gp` uses pure Rust COBYLA by relf in <https://github.com/relf/egobox/pull/110,https://github.com/relf/egobox/pull/113>
* `ego` as pure Rust implementation (`nlopt` is now optional) by relf in <https://github.com/relf/egobox/pull/112>
* `egobox` Python module: Simplify mixed-integer type declaration by relf in <https://github.com/relf/egobox/pull/115>
* Upgrade dependencies by relf in <https://github.com/relf/egobox/pull/114>
* Upgrade edition 2021 by relf in <https://github.com/relf/egobox/pull/109>
* CI maintainance by relf in <https://github.com/relf/egobox/pull/111>
* Bump actions/checkout from 2 to 4 by dependabot in <https://github.com/relf/egobox/pull/107>
* Bump actions/setup-python from 2 to 4 by dependabot in <https://github.com/relf/egobox/pull/108>