BUG FIX: Insert dummy gene

It could be the case that a peak occurs in every gene in a geneset
(chipenrich) or that every gene is 100% covered in a geneset (ce.ratio).

In the event that this happens, a "continuity_correction" gene is
created to help the gam(…) function converge.
Added QC Plot for ce.ratio method

For the ce.ratio method, it is helpful to see the relationship between
the log10(locus length) and the percentage of a locus covered by a
ChIP-seq peak.