OpenAtlas

Because automatic checks (executed via the admin area in this case) can be very useful, we added some for more recent features.

• Super/sub event checks
• Preceding/succeeding event checks
  After discussing it with Vera (thanks for the input and taking the time) I decided to only check for inconsistent begin dates.
  We never thought or talked about if it should be possible to chain overlapping events, so I will ignore end dates for this check for now.