🐛 set mappings_provided to list type so object is picklable

[davidhopkinson26]

fix for issue #257

Changes mappings_provided attribute to a list type rather than dict.keys() as the latter causes an error when pickling the transformer.

I have added tests that object is serialisable both to base_tests.py and the to test_DateTimeInfoExtractor.py (as this module has not yet been updated to the new testing framework)

These currently use a slightly hacky clean up using os.remove and a try except block to avoid persisting pickle files. I tried using the pytest tmpdir fixture instead but it didn't play well with joblib and kept giving me unicode errors when calling joblib.dump:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid start byte

Another approach to this test would be to directly check the dtypes of each of the transformers attributes to check they are serialisable objects i.e. one of number/ list/ string/ dict which i think should catch the issue too. Please mention in review if you think this would be better.

At on point i had the test loading the pickle and comparing to original object, however i removed this as comparing transformer objects with eq doesn't work and also because it was straying into unit testing joblib! If we really wanted to do this we would need to implement something to iterate over attributes and compare one by one.

[Decima2014]

Agree with the general approach of not testing the dtype of every single attribute here and comparing transformers between joblib dump and load. Would like to play around a bit more with using tmp path but if that doesnt work either then happy to move on

Thank you for picking up these changes

[Decima2014]

Happy with the changes, have approved, thanks again for picking this up :)