Minor: added default overall growth rate of 1 (for relative results)
Added comparing insertion densities between features/datasets; other.

NOTE: this was mostly done a while ago (a couple of weeks?) but not
finished or committed.

Added two functions to compare insertion densities and get pvalues:
between features in the same dataset, and same feature between datasets.
I think the statistics are right; both have FDR-adjustment.

Added _get_nice_feature_counts_data - currently not used anywhere!
Note to say BCBio.GFF library stopped working! Waiting on resolution.