Revisit checking functions

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.