NearestImputer Update : Remove median imputer when no null

[ClaireF57]

Use Case : When a column has no null when fitting NearestMeandResponseImputer

• Previously: have the possibility to have a median imputer
• Now: This triggers an error. Goal is that in case there is no null, the NearestMeandResponseImputer is not the right one to use but directly a median imputer.

Code update :

• tubular/imputers.py : removed median imputation
• tests/imputers/test_NearestMeandResponseImputer.py : removed the test concerning use case with median imputation , updated other test to remove default init concerning the use of median imputer as default
• example/imputers/NearestMeandResponseImputer.ipyng: removed the code associated to the use of median imputer in notebook and put the example to "deprecated"

[richardangell]

all looks good, thanks, just very minor tidying of github action file needed