Refactor nominal transformer input checks

[TommyMatthews]

For OrdinalEncodingTransformer and NominalToIntegerTransformer, we currently have to call a data check that from BaseTransformer.transform before you call check mappable rows from BaseNominalTransformer. BaseTransformer.transform is then called in full later in the method, leading to duplication of data checks. We would ideally refactor the code so that this isn't the case.

See this issue for more information

[TommyMatthews]

This issue is caused by the slightly confusing nature of the tubular BaseMappingTransformMixin and how it overwrites the transform method, similar to the comment on this issue: #146.