Smt

* 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

* fix bug in MFKPLS when <code>eval_noise</code> and <code>optim_var</code> are set to <code>True</code>