New enrichment method: ChIP-Enrich Ratio

ChIP-Enrich Ratio is an additional enrichment method whose model is:
goterm ~ ratio + spline, where ratio refers to the percentage of a
gene locus which is covered by a ChIP-seq peak. It is meant for use
with histone modifications or transcription factors with broader
ChIP-seq peaks.

Three new functions are implemented for this method:
1. assign_peak_segments(...) determines the overlap between
the peaks and gene loci in base pairs. Note, a peak overlapping more
than one locus has it's base pairs proportionally assigned.
2. test_gam_ratio(...) builds the model and performs the
enrichment.
3. plot_gene_versus_coverage(…) plots the log10 length of the gene locus
against the percentage covered by a peak. It is a part of the QC plots.

Two functions are modified:
1. num_peaks_per_gene(assigned_peaks,locusdef,…,ratio) will function as
with method=='chipenrich' when ratio==F. When ratio==T an additional
column is added to the peaks per gene table indicating the ratio of
the gene locus covered by a peak.
2. chipenrich(…) is modified to use assign_peak_segments(…) and
num_peaks_per_gene(...,ratio=T) when method=='ce.ratio'.