Revisit checking functions

[njtierney]

From #687

• Look at various "TODO" notes to identify places for refactoring wrt checking
• Consider consolidating/categorising checking functions in checkers.R.
  • e.g., Put all installation checking functions in checkers-installation.R, put inference related checking functions in checkers-inference.R.
• Review similar checking functions and see if they can be combined. We might not need so many custom checking functions
• Check test coverage to ensure that the checking functions are all covered. This provides another way to look at improving test coverage
• Revisit the error messages provided - could they provide more information?
• Ensure all checking functions are exported in module.