Egobox

* Automate Python package build and upload on Pypi from Github CI by relf in <https://github.com/relf/egobox/pull/104>
* Fix FullFactorial when asked nb iof samples is small wrt x dimension
by relf in <https://github.com/relf/egobox/pull/105>
* Make mixed-integer sampling methods available in Python by relf in <https://github.com/relf/egobox/pull/106>

* `gp`, `moe` and `egobox` Python module:
* Added Gaussian process sampling (97)
* Added string representation (98)

* `egobox` Python module:
* Change recombination enum to respect Python uppercase convention (98)

* Notebooks and documentation updates (97, 98, 99)

* `ego`:
* Infill criterion is now a trait object in `EgorSolver` structure (92)
* `Egor` and `EgorSolver` API: methods taking argument of type Option\<T\>
now take argument of type T (94)
* `EgorBuilder::min_within_mixed_space()` is now `EgorBuilder::min_within_mixint_space()` (96)
* `egobox-ego` library doc updated (95)

* `egobox` Python module: Upgrade to PyO3 0.18 (91)

* `ego`:
* Fix Egor solver best iter computation (89)

* `ego`:
* Make objective and constraints training in parallel (86)
* Lock mopta execution to allow concurrent computations (84)
* Fix and adjust infill criterion optimmization retries strategy (87)
* `moe`:
* Fix k-fold cross-validation (85)

* `ego`:
* Renaming `XType`, `XSpec` for consistency (82)
* Export history in optimization result (81)
* Use nb iter instead of nb eval, rename q_parallel as q_points (79)
* Warn when inf or nan detected during obj scaling computation (78)
* Parallelize constraint scales computations (73)
* Parallelize multistart optimizations (76)
* Handle GMM errors during MOE training (75)
* Handle possible errors from GMM clustering (74)
* Upgrade argmin 0.8.0 (72)
* Add mopta08 test case as example (71)
* Fix scaling check for infinity (70)
* Use kriging surrogate by default (69)