Resolve "Include images for PCA"

Closes #47

See merge request sis/courses/machinelearning-introduction-workshop!48

Resolve "Fix or remove comment on out-of-bag-error for Random Forests"

Closes #44

See merge request sis/courses/machinelearning-introduction-workshop!45

Resolve "Update installation instructions"

Closes #50

See merge request sis/courses/machinelearning-introduction-workshop!47

Resolve "Validation data in the time series regression"

Closes #57

See merge request sis/courses/machinelearning-introduction-workshop!44

# Conflicts:
#   07_regression.ipynb

Resolve "Clarify scaling description and pipelines"

Closes #46

See merge request sis/courses/machinelearning-introduction-workshop!43

# Conflicts:
#   CONTRIBUTING.md

Resolve "Script 4: fix references and typos in exercise section"

Closes #54

See merge request sis/courses/machinelearning-introduction-workshop!42

Resolve "Don't store notebook output cells"

Closes #56

See merge request sis/courses/machinelearning-introduction-workshop!41

minor updates

See merge request sis/courses/machinelearning-introduction-workshop!37