zoomfunction for ld-plot is working

Now it is possible to zoom into a desired region of the arc plot and the ld-plot only displays the zoomed region. Issues arised:
- also the snps with an interaction degree 0 appear. ( I discussed this with cheng this is a general issue in the json parsing) 
- and I have to implement a labeling for the ld plot or at least an arrow, which indicates the position of the snp in the arc plot.