save_models fix for #125 (#127)

[apaleyes]

• Updated GPModel to save full parameter name list, added simple save tests, and initialised self.model_parameters_iterations in init

[codecov-io]

Codecov Report

Merging #140 into master will increase coverage by 0.63%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   76.11%   76.75%   +0.63%     
==========================================
  Files          91       92       +1     
  Lines        4112     4160      +48     
==========================================
+ Hits         3130     3193      +63     
+ Misses        982      967      -15

Impacted Files	Coverage Δ
GPyOpt/core/bo.py	57.97% <100%> (+4.53%)	⬆️
GPyOpt/testing/core_tests/test_save.py	100% <100%> (ø)
GPyOpt/models/gpmodel.py	48.44% <100%> (+1.86%)	⬆️
GPyOpt/methods/bayesian_optimization.py	88.63% <0%> (+4.54%)	⬆️

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