Lots of work on the JaccardCalculator:

* Added threshold property, that makes it possible to skip the insanely small values that sometimes occur. Less values equals less work equals more speed and less memory usage. NOTE: need to modify property editor to allow entering smaller values!
* Changed format of work packages, and especially the results.
* Changes in thread handling
* Changes in handling of work packages

We could probably improve performance further if we change the handling of the results. Make the packages bigger again, and store the results locally. Only at the end, the results can then be merged. This would simplify the setup of the class, as we no longer need a thread to handle the results. That thread can then also no longer be in the way of other threads.