Modify CaseSection class to be container of case generation classes

[mkolisnyk]

Currently CaseSection generate method is too big and it is not really extensible. E.g. if I want to add some generation scenario I need to update this class with additional methods. Eventually this class will become too big. At the same time case generation can be wrapped with common interface and CaseSection class can be just the container for it.
The aim of the task is to split entire case generation functionality into 3 classes (for positive/negative and unique values scenarios) one per each test type. The CaseSection itself will just contain list of classes and method which iterates through those classes