Uniform method to categorize versions

A function to categorize versions into affected and safe versions is present in multiple importers such as
https://github.com/nexB/vulnerablecode/blob/e9943b014f5f07b970988f910b2aaebcec0a6e58/vulnerabilities/importers/github.py#L320-L338

https://github.com/nexB/vulnerablecode/blob/e9943b014f5f07b970988f910b2aaebcec0a6e58/vulnerabilities/importers/apache_httpd.py#L129-L148

https://github.com/nexB/vulnerablecode/blob/e9943b014f5f07b970988f910b2aaebcec0a6e58/vulnerabilities/importers/istio.py#L66-L83

They are present with different names and sometimes even with different parameters. This is confusing and hard to document.
Imo, we should create an abstract method in the `DataSource` class with a uniform name and parameters. This would also help me build the small documentation I'm working on for vulnerablecode better.