Fixed strategy

[bjherger]

Addressing #161 , bringing CategoricalImputer terminology to be consistent with SimpleInputer

• Refactored so that terminology is consistent
• Updated documentation to correctly reflect constant mode
• Update default fill_value to '?', consistent with SimpleInputer

[devforfu]

@bjherger I think that everything is fine, though probably we could add an additional test to replace the excluded one, i.e., test_missing_replacement, to make sure that the constant strategy uses the ? symbol when fill_value is not provided. Something like:

def test_default_fill_value_for_constant_strategy():
    data = [None, 'b', 'c']
  
    imputer = CategoricalImputer(strategy='constant')
    Xt = imputer.fit_transform(X)
    
    assert imputer.fill_ == '?'
    assert (data == [imputer.fill_, 'b', 'c']).all()

[bjherger]

@devforfu Great point! I've added another test, based on your suggested code and test_custom_replacement().

Please review and let me know your thoughts.

[devforfu]

@bjherger Ok, great! Yes, thank you for the PR, I think we can merge your changes into master.