A to-do list for the Gviz package

General
   o check the plotting with small coordinates, do all track types align?
   
AnnotationTrack/GeneRegionTrack
   o speed up .prepareGene (may need a C code module)
   o deal with additional annotation features (UTRs, etc.)

DataTrack/AnnotationTrack => SNPsTrack?
   o issue: #5
   o new track type to handle LD information
   o ideally as heatmap with defined depth (range)
   o check following: snpStats (data structure), ldblock, gwascat 
   o similar thing already implemented in LDheatmap
