Modularize the code fo nested_enrich class <!-- This issue will be created in repo nexomis/fctBio (https://github.com/nexomis/fctBio). Changing this line has no effect. -->

[jfouret]

The class nested_enrich is quite big and could be modularized. The idea is to create a new file for large method with non-exported function.
Note that we cannot put R files into directory (https://stat.ethz.ch/pipermail/r-devel/2009-December/056024.html) (https://stat.ethz.ch/pipermail/r-devel/2009-December/056024.html).