#16 data processing for sprints

[Klaus]

It sometimes happens that the enddate of a sprint is also set for another (previous) sprint, but this bug is not easy to reproduce.
If it happens the name of the last edited sprint overrides the name of the sprint, which enddate was influenced unintendently.
Then the data seem to be "coupled" in this sense: If the user changes the datum of one sprint, the equivalent datum of the other ist changed too.
A restart of JAST resets this misbehaviour.