Smt

* 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>

* add MFKPLS application (193, thanks m-meliani )
* add MFKPLSK application (198, thanks m-meliani )
* add parallel EGO with qEI criterion (202 , 199, 190, thanks EmileRouxSMB and rruusu )
* add notebook tutorial for EGO method
* fix kriging based surrogate bug (200 )
* fix full factorial sampling weights and clip options (197 )
* fix sklearn 0.22 cross_decomposition warning (196 )
* next releases > 0.5.x will likely **drop Python 2.7 support**

* fix EGO bugs (192)