topAnatData S4 class validity

[julien-roux]

It is a good practice to validate the objects using a validity method (setValidity for S4 classes).

In topAnat.R we extend from topGOdata class but do not perform such a validity check. As far as I can tell, topGO doesn;t do it either... (see the AllClasses.R files in their package).

A good point (more elegant) is that it could replace/gather some checks that are now made in the beginning of the topAnat function...