Naming consistency

[thomaszwagerman]

Adds:

• adds examples and return statements to documentation, closes adding examples to function documentation #6
• adds input checks to all functions
• improves consistency in naming 'new' 'current' and 'old' 'previous', closes improve consistency in function variable naming and documentation #7
• replaces print() statements with cli:cat_print(), closes change all instances of cat() or print() to cli::*() calls #8