🚧 update imputation module (#61)

* 🎨 format and docstrings

* 🎨 fix format of links and listing

* ♻️ refactor: imputation_KNN

- add examples to api examples

* 🚧 update impl. of the downshifting function

- use slightly newer API
- many things left to do

* ✅ add basic testing of module

* ⚡ avoid transposing the data

- access across rows is also expensive, but not all data needs to be moved twice

* ✅ extend nomal_distribution_test with two random states

* 🐛 fix bugs with groups and no groups passed, return shapes

- update examples
- needs further refinement

* 🎨 extend example with plots, more explanations

* 🎨 add type hints

* 🎨 typos and example from pimms

* 🎨 apply typo and design suggestion from copilot

* 🐛 add test for mixed imputation and fix bug

* 🎨 update tutorial

Co-authored-by: Copilot <copilot@github.com>

* 🎨 format tutorial

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Co-authored-by: Copilot <copilot@github.com>

Author: enryH