Refactor df_check.py - Githubissues

An attempt at clarifying the workings of df_check.py. Main changes include:

Refactored original df_check-function into many smaller functions: entrypoint now is the run-function
Changed to use of a single dataframe instead of a list of dataframes: looping over dataframes can be done by the user, if needed
Added ValidationSettings-dataclass for storing all settings related to the validation of a single dataframe
Added common.py for shared classes and functions
Added some unit tests
Changed contents of README.md
Renamed some parameters (mainly spark_session and dq_rules_dict)

Amsterdam / dq-suite-amsterdam