make CategoricalImputer not raise an error with multimodal variables

[solegalli]

In the current implementation, if a categorical variable has 3 categories with equal frequency, the transformer will raise an error.

As we move away from raising errors and instead allowing the transformers to work and inform the user instead, I'd like to add a parameter to govern whether the transformer should error or proceed, and if it proceeds, it should output a message saying that it has multiple modes, and that it will use the first category found as the most frequent.