Correct bug (divide by 0) in uq normalization.

Some times when one library was orders of magnitude larger than another it
would have much more elements (genes, exons, etc) with non-zero values. In
these cases the index of the 25th percentile element would result in getting a
0 value from the smallest table (smallest library). This would create a
division by 0 error during normalization. Now the 25th percentile index is
calculated from the smallest table.
Rename UQ norm to compare_counts and add plotting.