add feature/score for survey time dimension

[andreaskutka]

Problem:
Interviewer behaviour is expected to change as they familiarise themselves with the questionnaire, Survey Solutions, etc. Interviews created earlier during a survey may differ somewhat to those towards the end.

We have explored the following features:

• days since start of survey (take minimum answer set)
• sequential number of interview (sorted by time) by interviewer
  We have not found any conclusive patterns yet. In our experiment, all artificially created fakes were produced after the real interviews.

Solution:
Investigate further once more testing data is available.
Maybe allow user to specify the expected number of days of the survey, so days since start can be expressed relatively.