[Scenario] Find end time in collections

Timeline objects should be able to display
Activities found in Collections in the
Timeline View, nasa/MCT-Plugins#37. Since
Timeline View uses the Timeline component's
end time to determine the displayed time scale,
this means that the Timeline component should
also investigate collections in order to
determine what its end time is.
Merge pull request #65 from VWoeltjen/mctPlugins37

[Scenario] Find end time in collections