extend drop psi functionality to categorical variables (#664)

* paste changes on psi transformer

* tidys docstrings, add detail, fix bugs

* add checks for pvalue

* space

* black and isort

* remove unnecesary intermediate variable

* rephrase comment

* space docstring

* fix typo

* fix style

* add option to select all or only numerical variables, fixed number of bugs

* fix style

* fix type

* workaround to pass general tests

* expand coverage

Author: solegalli