Smt

* Noise API changes for Kriging based surrogates (276, 257 thanks anfelopera):
- add a new tutorial notebook on [how to deal with noise in SMT](https://github.com/SMTorg/smt/blob/master/tutorial/SMT_Noise.ipynb)
- rename <code>noise</code> as <code>noise0</code> option and is now a list of values
- add option <code>use_het_noise</code> to manage heteroscedastic noise,
- improve noise management for MFK (different noise by level),
- add option <code>nugget</code> to enable the handling of numerical instabilitily
- [matern kernel documentation](https://smt.readthedocs.io/en/latest/_src_docs/surrogate_models/krg.html#kriging)
* Add <code>predict_variance_derivatives</code> API (256 , 259 thanks Paul-Saves)
- add spatial derivatives for Kriging based surrogates
- fix respect of parameters bounds in Kriging based surrogates
* Notebooks updates (262, 275 thanks NatOnera, 277 thanks Paul-Saves )
- [SMT tutorial](https://github.com/SMTorg/smt/blob/master/tutorial/SMT_Tutorial.ipynb)
- [SMT EGO tutorial](https://github.com/SMTorg/smt/blob/master/tutorial/SMT_EGO_application.ipynb)
- [SMT Mixed Integer tutorial](https://github.com/SMTorg/smt/blob/master/tutorial/SMT_MixedInteger_application.ipynb)
* Kriging based surrogates refactoring (261 thanks anfelopera)
- inheritance changes: MFKPLS -> MFK, KPLSK, GEKPLS -> KPLS
- improve noise options consistency
- improve options validity checking
* Code quality (264, 267, 268 thanks LDAP):
- use of abc metaclass to enforce developer API
- type hinting
- add 'build system' specification and requirements.txt for tests, setup cleanup

* allow noise evaluation for Kriging based surrogate (251)
* fix optimizer bounds in Kriging based surrogate (252)
* fix MFK parameterization by level (252)
* add <code>random_state</code> option to LHS sampling method for test repeatability (253)
* add <code>random_state</code> option to EGO application for test repeatability (255)
* cleanup tests (255)

* add Marginal Gaussian Process surrogate models(236, thanks repriem)
* add Matern kernels for kriging based surrogates (236, thanks repriem)
* add gradient based optimization for hyperparameters in kriging based surrogates: new <code>hyper_opt</code> option to specify TNC Scipy gradient based optimizer, gradient-free Cobyla optimizer remains the default. (236, thanks repriem)
* add <code>MixedIntegerContext</code> documentation (234 )
* fix bug in <code>mixed_integer::unfold_with_enum_mask</code> (233 )

* Application: [Mixed integer sampling methods and surrogates](https://smt.readthedocs.io/en/latest/_src_docs/applications/mixed_integer.html) (#229)
* handling of categorical and integer variables in [Kriging](https://smt.readthedocs.io/en/latest/_src_docs/surrogate_models/krg.html#usage-with-mixed-variables) (219, thanks Paul-Saves)
* handling of categorical and integer variables in [EGO optimizer](https://smt.readthedocs.io/en/latest/_src_docs/applications/ego.html#usage-with-mixed-variable) (220, thanks Paul-Saves)
* remove initial doe returned value from EGO optimize method (224)
* drop Python 2.7 (215, 227)
* fix MFK variance computation (211)
* fix MOE experts selection (223)
* fix MOE RMTS usage (225)
* fix QP as used in run_examples (226)

* fix bug when <code>eval_noise</code> is <code>True</code>

* add <code>packaging</code> dependency in setup