Add general pre processing that can be overriden on demand

Extracted task from issue #51:

• "add general pre processing, which happens before any analysis is started
  • "sort out empty / bad entries, generate descriptive statistics, ..."
  • "by argument, config file, (hard coded), ..."
  • "document what was done (replacing values, dropping entries, ...)"

Here we want to develop a configurable general pre-processing (e.g. give a list of mandatory questions that need to be answered to mark the whole answers of a participant as complete) that can be overridden on demand.