Added a new file type that can be tested: GenericFile
Added a new file type that can be tested: GenericFile

 - Added `GenericFile` class to `compliance_checker/base.py`:
   - it has one method: `filepath()` - returning the file path given on instantiation.
 - Modified `load_local_dataset` method of `CheckSuite` in `compliance_checker/suite.py`:
   - when tests for other supported file types have been done then a check is
     done to see if the string represents an existing file. If so, then the
     method returns an instance of `GenericFile`
 - 3 tests for `GenericFile` added in:
   - `compliance_checker/tests/test_base.py`
   - `compliance_checker/tests/test_suite.py`
 - 1 new empty test file added for the tests:
   - `compliance_checker/tests/data/non-comp/empty.file`
 - `test_requirements.txt` updated to include `httpretty`

 - The `GenericFile` can now be used as one of the supported datasets in the
   `supported_ds` property of a Check class.

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