Moved openReport to the Report class

Moved openReport to the Report class such that it got accessible from the bucket class without importing the main class
Creation of getLastTrace

Adding the getLastTrace method to the Report class. And replacing all the calls to getTraces().get(0) by it.

Last trace was chose since prof. Monperrus mentioned the last trace to be the most recent and thus most usfull one.
Added a unit test class for the crashGraph.enteties

This unit test tests the loading of 1524.json report and the similarity of 1755.json and 1759.json
Constructor of Bucket added

A constructor was added to Bucket, which tries to minimalize the saved data to the strict nescessary for later.