Convert tidy_corrm() to S3 method

[r-link]

Idea:

• Create new ggproto class named TidyCorrm containing a set of functions that break up the data cleanup and preparation into resonable portions,
• Define a generic for method tidy_corrm(),
• Create methods for tidying up different classes of objects based on TidyCorrm object (current function should be reflected in tidy_corrm.default(),
• Prepare documentation that makes it easy to create new methods for different object types (e.g. Stan output etc.) by extending the TidyCorrm() class.