Refactor imputation module

[enryH]

Summary

Fixes #

...

List of changes proposed in this PR (pull-request)

• ...
• ...

Checks

• Look at CONTRIBUTING.md for guidance on how to contribute.

In case you add a new module or update one, please check the following tasks:

• The folder names defines the name of the subpackage (module)
• Add the user-facing functions to the __init__.py in the new folder, so that
  they are available when the subpackage is imported.
• Create Pandera schema in a file with subpackage name in the src/acore/types folder.
  Optimal is to have only one output schema of results per subpackage or module.
• Add a relatively small and public dataset to the data folder, or reuse an existing one for testing
• Create an api example jupyter notebook in the docs/api_examples_module folder with that data
• Use jupytext to sync the Jupyter notebook with a Python script
• Update index.md file in the docs folder with the new example
• Create test script in the /tests folder with the name of the subpackage or module
  using pytest or unittests to test your new functionality.