Compare factors in differential expression

[eweitz]

This enables comparing all pairs of experimental factors in differential expression (DE) datasets.

Previously in #184, only two factors could be compared, e.g. "space flight" and "ground control". This was inadequate for DE datasets with more than two factors, e.g. "space flight", "ground control", and "vivarium control".

See the updated "Differential expression" example:

For datasets with many factors, naively encoding all possible pairwise combinations in the same Ideogram.js annotation JSON file causes very large file sizes. The number of pairwise comparisons in a dataset with n factors is given by n! / (2! * (n - 2)!).

Left unoptimized, for 20 factors this could yield 30-50 MB JSON files and load times of 10-20 seconds with average U.S. bandwidth -- an unacceptable user experience.

To address that, these changes also split DE datasets with n factors into n JSON files. So instead of a 20-factor DE dataset requiring a file size of 190x, it requires a file size 20x. This graphs shows the relative rates of growth of the old and new data structures.

[coveralls]

Coverage increased (+1.0%) to 90.55% when pulling 97e8a00 on deg-compare-factors into aab4ffb on master.