Caching of Differential Expressed Genes

[madiganz]

In the R Code, the Differential Expression (DE) calculations are cached so that they don't need to be recomputed for already computed cluster pairs.

• Determine if we want to do caching
• Determine how to do the caching

[wbwakeman]

Let's get a first version out and working, and then evaluate this based on the need when performance becomes an issue