Standardize "_3f" or "3f"

[julistanley]

rendseq/rendseq/make_wig.py

Line 36 in 3bf0d20

wig_file = "".join([prefix, track, ".wig"])

This line writes a file as "filename3f.wig", etc, but later in the pipeline they expect "filename_3f.wig", etc. Just change to wig_file = "".join([prefix, "_", track, ".wig"]) to standardize.