Restructure SQLiteDataConverter, and align with DataConverter class

Created by: asogaard

Following PR #18 these two classes are still not fully aligned, e.g. not all base class methods are used in the inheriting class (e.g., _process_file vs. _parallel_extraction); there is some duplication in how data is being written to SQLite databases; etc. Are you okay with me working on addressing this, @RasmusOrsoe, and proposing a few code tweaks?