Incorporate documentation edits from Schloss Lab meeting

[kelly-sovacool]

This fixes just a few of the things mentioned:

• Resolves Preprocessing Data Vignette text correction #228
• Resolves Continuous data normalization methods #229
• Resolves Edit hyperparameter tuning vignette #230 @BTopcuoglu

I merged the cran-release branch here which also speeds up vignette building.

[codecov-io]

Codecov Report

Merging #232 (32d81e4) into master (f50202d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #232   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files          12       12           
  Lines         788      788           
=======================================
  Hits          755      755           
  Misses         33       33           

Impacted Files	Coverage Δ
R/performance_metrics.R	92.30% <ø> (ø)
R/preprocess_data.R	100.00% <ø> (ø)
R/run_ml.R	100.00% <ø> (ø)

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