add hartigan's dip test and tukey test. ref #22, #28, #35

[tompollard]

This pull request:

• Hartigan's Dip Test for multimodality (https://en.wikipedia.org/wiki/Multimodal_distribution) as response to #22.
• Tukey's rule for outliers (https://en.wikipedia.org/wiki/Outlier#Tukey's_fences) in response to #35.
• Highlights potential issues with warnings/remarks below the table.
• Adds a test to check whether tableone is created correctly when columns are not specified as an argument.
• Some tidying of the tests