Align processor architecture

[djkhl]

While working through the Logprep Architecture some inconsistencies in the processors were found. This should be fixed to improve ongoing maintenance and future developments.

Following points should be revised:

• all processors should use the same functionality to read from the events (e.g get_dotted_field_value method from Helper module)
• all processors should use the same functionality to write to events (e.g write_to_field method form processor module)
• all processors should handle missing fields the same way (e.g handle_missing_fields from FieldManager)
• align return values form Pseudonymizer Predetector and SelectiveExtractor (see refactor code of pseudonymizer, pre_detector and selective_extractor #323)
• revise processors like clusterer so that they don't have to override the process method