Add `date-fns` wrapper

As !61 kind of got out of hand another iteration with less changes.

What I did:

• Rename DateString to CalendarDateString to emphasize it only handles calendar dates and ignores hours etc.
• Wrap date-fns functions in date helper file (which now understand both date and ISO8601 strings)
• Reworked one function in BasicDataViewService to get rid of some functions in CalendarDateString
• Added documentation
• Added functions in DateString calling the helper functions in date and transorming them to days.

Questions:

• How do you feel about the naming in CalendarDateString?
• Do the added docs help?

Discussion: I feel we should use the Date object where we want to do things with dates - e.g. adding days etc. We only should use the CalendarDateString where we explicitely need a string representation of calendar dates.