Rearrange function files

[drsimonj]

There are three main types of functions:

• Those that alter internal information but retain square matrix structure, and thus the cor_df class (e.g., rarrange, shave)
• Those that reshape the square matrix structure (e.g., rselect, rgather)
• Those used for generating output (e.g., rplot)

Each of these sets of functions should be placed in separate and appropriately named R files.

[github-actions]

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