Increase robustness of generated pipelines

[epeters3]

Resolves #48. Additions in this PR:

• Switch to using 38_sick as test data set (has more edge cases)
• Use new version of d3m.primitives.data_preprocessing.random_sampling_imputer.BYU that can handle all nan columns
• Simplify way to add hyperparameters to pipeline steps
• Remove extra steps on ensembles
• Change semantic types and one hot encode the output of preliminary classifiers (Note: requires random pipeline generator to not be able to make structure batches)
• One hot encode all categorical and ordinal attributes in the common preliminary pipeline steps

[codecov-io]

Codecov Report

Merging #72 into develop will decrease coverage by 0.33%. The diff coverage is 92.42%.

@@             Coverage Diff             @@
##           develop      #72      +/-   ##
===========================================
- Coverage    65.72%   65.39%   -0.34%     
===========================================
  Files           26       26              
  Lines         1666     1653      -13     
===========================================
- Hits          1095     1081      -14     
- Misses         571      572       +1

Impacted Files	Coverage Δ
test/test_pipeline_builder.py	97.97% <ø> (-0.05%)	:arrow_down:
experimenter_driver.py	28.37% <ø> (-0.49%)	:arrow_down:
test/test_random_pipelines.py	100% <100%> (ø)	:arrow_up:
test/test_pipeline_generator.py	100% <100%> (ø)	:arrow_up:
test/test_ensembling_pipelines_generation.py	96.49% <100%> (ø)	:arrow_up:
test/test_cli.py	100% <100%> (ø)	:arrow_up:
experimenter/constants.py	100% <100%> (ø)	:arrow_up:
experimenter/experiments/straight.py	82.85% <100%> (ø)	:arrow_up:
test/test_ensembling_pipeline_runs.py	94% <100%> (-4.12%)	:arrow_down:
experimenter/experiments/ensemble.py	75.22% <100%> (-0.65%)	:arrow_down:
... and 5 more

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