Rename `get_suites()` to `get_checkers()` for interface with generator plugins
Rename `get_suites()` to `get_checkers()` for interface with generator plugins

The rest of the code refers to the check classes as  *checkers*, and the
check *suite* is a separate class that knows about all the checkers.

Update my changes to be consistent with this.

aodn/compliance-checker,ocefpaf/compliance-checker,lukecampbell/compliance-checker,ioos/compliance-checker