No data checks in MappingTransformer transform

[TommyMatthews]

MappingTransform.transform(X) tries to do things to X before actually checking that it is a dataframe and has the required columns.

I think the best solution would be to call the BaseTransformer.transform method at the start of transform, which you can do with X = super(BaseMappingTransformer, self).transform(X).

[TommyMatthews]

On track to be fixed when mapping transformer is brought into the generic testing framework by the addition of a BaseTransformer.Transform call in this PR:
#171

However, this leaves the issue that there are now two calls to BaseTransformer.transform (one explicit one implicit) in the MappingTransformer.transform method. I think this best solved my changing BaseMappingTransfomMixin - either by declaring it redundant or changing the name of it's transform method, allowing the BaseTransformer.transform method to be replaced by a super transform method and making this the only call to base transformer transform.