Handle placeholder nodes in wiki pathways diagrams

[bsantan]

Example in WP4538_datanodeDF.csv
"placeholder
 
miR-135a-5p
miR-17-5p
miR-29b
miR-124
miR-519a","PTPRD","id116bdc57","i","#000000"

The goal is to interpret this as each node in the list being linked to the target/source node:
miR-135a-5p - STAT3 and
miR-135a-5p - PTDRD