Which metadata column to export as scoreExports a GInteractions object to BED12 format, and writes to a specified file. If filename is not specified,
then a data.frame containing the information is returned.
Bed12 files provide a method for visualising interactions, it is not a good format for storing all of the data associated
with an interaction dataset, particularly for trans-chromosomal interactions, which can only be stored in the bed12 names
field.