Consistent args and base transformers

[TommyMatthews]

This PR:

• sets up the generic testing for columns str and list transformers and adds BaseTransformer and DataFrameMethodTransformer (all transformers from base.py)
• cleans the test files for these transformers (including writing some new behaviour tests to replace implementation tests)
• Removes columns = None as an option from BaseTransformer (it was default), off the back of this:
  a) removed columns_set_or_check method from package as this is now redundant
  b) fixed ~60 failing unit tests that instantiated transformers without a columns argument (see note below)
  c) updated the example notebook to reflect the removal of this functionality
• Changed new_column_name to new_column_names in DataframeMethodTransformer to match standardisation going forward
• Fixed the tests that the above broke (see note below)

Note on existing implementation tests outside base test suite:
When going to fix broken tests I found that a lot of them were implementation tests. In order to maintain a sensible scope for this feature, my policy was:
If the test is in a test file for one of the transformers I am moving to generic testing: fix it if it isn't covered by generic testing.

Elif the test is/will be covered by the base behaviour tests: delete it - it will be covered at some point once generic testing gets to this module, and until then it's not really adding any value as an implicit test.

Elif the behaviour this implementation test is trying to implicitly test for will not be covered by the base behaviour tests: fix it quickly (with an xfail in mind but didn't need to) and mark testing this behaviour as TODO. Include bringing the test suite up to best practice standards as part of the scope of moving a transformer over to generic testing. Will include in process doc