Encoding consistency with imputations

[nicolassidoux]

Description of feature

Imputation functions require the data to be encoded. We have two different strategies in ehrapy to deal with that:

• Throw an exception if the given AnnData is not properly encoded
• Encode silently and perform the imputation

The second option is problematic because the user cannot control how the data should be encoded.
For the sake of consistency across all the imputation functions, we should modify the imputations that silently encode the data to make it throw an exception instead. We consider it's the user responsability to properly encode his data according to his needs.