commented out code with compiler errors
more changes to preprocessing classes

* reduced number of similar constructors
* added static preview methods
* added constructor that adds distances to existing graph
* cleaned up the code a bit
defined constants for labels
slight changes to Reconstruction class

Apart from making two methods public, I changed the type of returngraph.
This is only a temporary fix! We should think about what exactly the
return type of the metric graph reconstruction should be.
worked on visualization

* added main window
* added Canvas superclass