Rename visualize_tandem_pairs.py to _by_chr.py
tandem_dupes_all_pairs function does not require tss_data input, so it
was removed in tandem_data.py and visualize_tandem_pairs_by_chr.py.

Init commit: visualize_tandem_pairs_genome.py: plots tandem duplication
events on the genome scale. 

jplots.py: added vertical span function to accommodate
visualize_tandem_pairs_genome.py