Further unification and simplification to the pipeline building API

[epeters3]

Resolves #70. Further simplifies how pipelines are built in the repo. Also pulls more logic specific to pipeline building into the EZPipeline class.

[codecov-io]

Codecov Report

Merging #71 into develop will increase coverage by 0.05%. The diff coverage is 89.34%.

@@             Coverage Diff             @@
##           develop      #71      +/-   ##
===========================================
+ Coverage    65.66%   65.72%   +0.05%     
===========================================
  Files           27       26       -1     
  Lines         1701     1666      -35     
===========================================
- Hits          1117     1095      -22     
+ Misses         584      571      -13

Impacted Files	Coverage Δ
experimenter/experimenter.py	67.96% <0%> (+1.8%)	:arrow_up:
test/test_pipeline_builder.py	98.01% <100%> (+1.05%)	:arrow_up:
experimenter/experiments/random.py	80.55% <100%> (-6.18%)	:arrow_down:
experimenter/constants.py	100% <100%> (ø)	:arrow_up:
experimenter/experiments/metafeatures.py	45.45% <25%> (ø)	:arrow_up:
experimenter/experiments/straight.py	82.85% <66.66%> (+12.4%)	:arrow_up:
experimenter/experiments/ensemble.py	75.86% <84%> (-3.48%)	:arrow_down:
experimenter/pipeline_builder.py	97.14% <97.91%> (-0.9%)	:arrow_down:
...hundred_plants_margin_solution/modules/__init__.py

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[epeters3]

Unit tests are on their way.